Language: C++

#include <bits/stdc++.h>
using namespace std;

/* REPmacro */
#define REP(i,n) for(int i=0,i##_len=(n);i<i##_len;++i)
#define REPR(i,n) for(int i=((int)(n)-1);i>=0;--i)
#define FOR(i,m,n) for(int i=(int)(m);i<(int)(n);++i)

/*alias*/
#define ALL(x) (x).begin(),(x).end()
#define SZ(x) ((int)(x).size())
#define F first
#define S second
#define PB push_back
#define MP make_pair 

#define y0 y3487465
#define y1 y8687969
#define j0 j1347829
#define j1 j234892
#define next asdnext
#define prev asdprev

/*using*/
using ll = long long;
using ull = unsigned long long;
using unsi = unsigned;
using vi = vector<int>;
using vvi = vector<vi>;
using vvvi = vector<vvi>;
using pii = pair<int, int>;
using db = double;
using plex = complex<double>;
using vs = vector<string>;

/*template*/
template<class T>inline bool amax(T &a, const T &b) { if (a<b) { a=b; return 1; } return 0; }
template<class T>inline bool amin(T &a, const T &b) { if (b<a) { a=b; return 1; } return 0; }

/* func */

/*struct*/
struct aaa{
    aaa(){
        cin.tie(0); ios::sync_with_stdio(0); cout<<fixed<<setprecision(20);
    };
}aaaaaaa;

/* const */
const int INF = 1001001001;
const ll LINF = 1001001001001001001ll;
const int MOD = 1e9 + 7; //10^9
const db EPS = 1e-9;
const int dx[] = {0, 1, 0, -1, 1, -1, 1, -1}, dy[] = {1, 0, -1, 0, 1, -1, -1, 1};

//#define int ll
//#define int long long

signed main()
{
  string s; cin >> s;
  auto ssize = s.size();
  set<string> t;
  REP(i,ssize){
    auto c = s[i];
   t.insert(c);
  }
  
  //setを使いたい誰か教えてください！！！！！
  
  
  return 0;
}

Compiler output:
a.cc: In function 'int main()':
a.cc:66:12: error: no matching function for call to 'std::set<std::__cxx11::basic_string<char> >::insert(char&)'
   66 |    t.insert(c);
      |    ~~~~~~~~^~~
In file included from /usr/include/c++/14/set:63,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:158,
                 from a.cc:1:
/usr/include/c++/14/bits/stl_set.h:568:9: note: candidate: 'template<class _InputIterator> void std::set<_Key, _Compare, _Alloc>::insert(_InputIterator, _InputIterator) [with _Key = std::__cxx11::basic_string<char>; _Compare = std::less<std::__cxx11::basic_string<char> >; _Alloc = std::allocator<std::__cxx11::basic_string<char> >]'
  568 |         insert(_InputIterator __first, _InputIterator __last)
      |         ^~~~~~
/usr/include/c++/14/bits/stl_set.h:568:9: note:   candidate expects 2 arguments, 1 provided
/usr/include/c++/14/bits/stl_set.h:511:7: note: candidate: 'std::pair<typename std::_Rb_tree<_Key, _Key, std::_Identity<_Tp>, _Compare, typename __gnu_cxx::__alloc_traits<_Alloc>::rebind<_Key>::other>::const_iterator, bool> std::set<_Key, _Compare, _Alloc>::insert(const value_type&) [with _Key = std::__cxx11::basic_string<char>; _Compare = std::less<std::__cxx11::basic_string<char> >; _Alloc = std::allocator<std::__cxx11::basic_string<char> >; typename std::_Rb_tree<_Key, _Key, std::_Identity<_Tp>, _Compare, typename __gnu_cxx::__alloc_traits<_Alloc>::rebind<_Key>::other>::const_iterator = std::_Rb_tree<std::__cxx11::basic_string<char>, std::__cxx11::basic_string<char>, std::_Identity<std::__cxx11::basic_string<char> >, std::less<std::__cxx11::basic_string<char> >, std::allocator<std::__cxx11::basic_string<char> > >::const_iterator; typename __gnu_cxx::__alloc_traits<_Alloc>::rebind<_Key>::other = std::allocator<std::__cxx11::basic_string<char> >; typename __gnu_cxx::__alloc_traits<_Alloc>::rebind<_Key> = __gnu_cxx::__alloc_traits<std::allocator<std::__cxx11::basic_string<char> >, std::__cxx11::basic_string<char> >::rebind<std::__cxx11::basic_string<char> >; typename _Alloc::value_type = std::__cxx11::basic_string<char>; value_type = std::__cxx11::basic_string<char>]'
  511 |       insert(const value_type& __x)
      |       ^~~~~~
/usr/include/c++/14/bits/stl_set.h:511:32: note:   no known conversion for argument 1 from 'char' to 'const std::set<std::__cxx11::basic_string<char> >::value_type&' {aka 'const std::__cxx11::basic_string<char>&'}
  511 |       insert(const value_type& __x)
      |              ~~~~~~~~~~~~~~~~~~^~~
/usr/include/c++/14/bits/stl_set.h:520:7: note: candidate: 'std::pair<typename std::_Rb_tree<_Key, _Key, std::_Identity<_Tp>, _Compare, typename __gnu_cxx::__alloc_traits<_Alloc>::rebind<_Key>::other>::const_iterator, bool> std::set<_Key, _Compare, _Alloc>::insert(value_type&&) [with _Key = std::__cxx11::basic_string<char>; _Compare = std::less<std::__cxx11::basic_string<char> >; _Alloc = std::allocator<std::__cxx11::basic_string<char> >; typename std::_Rb_tree<_Key, _Key, std::_Identity<_Tp>, _Compare, typename __gnu_cxx::__alloc_traits<_Alloc>::rebind<_Key>::other>::const_iterator = std::_Rb_tree<std::__cxx11::basic_string<char>, std::__cxx11::basic_string<char>, std::_Identity<std::__cxx11::basic_string<char> >, std::less<std::__cxx11::basic_string<char> >, std::allocator<std::__cxx11::basic_string<char> > >::const_iterator; typename __gnu_cxx::__alloc_traits<_Alloc>::rebind<_Key>::other = std::allocator<std::__cxx11::basic_string<char> >; typename __gnu_cxx::__alloc_traits<_Alloc>::rebind<_Key> = __gnu_cxx::__alloc_traits<std::allocator<std::__cxx11::basic_string<char> >, std::__cxx11::basic_string<char> >::rebind<std::__cxx11::basic_string<char> >; typename _Alloc::value_type = std::__cxx11::basic_string<char>; value_type = std::__cxx11::basic_string<char>]'
  520 |       insert(value_type&& __x)
      |       ^~~~~~
/usr/include/c++/14/bits/stl_set.h:520:27: note:   no known conversion for argument 1 from 'char' to 'std::set<std::__cxx11::basic_string<char> >::value_type&&' {aka 'std::__cxx11::basic_string<char>&&'}
  520 |       insert(value_type&& __x)
      |              ~~~~~~~~~~~~~^~~
/usr/include/c++/14/bits/stl_set.h:548:7: note: candidate: 'std::set<_Key, _Compare, _Alloc>::iterator std::set<_Key, _Compare, _Alloc>::insert(const_iterator, const value_type&) [with _Key = std::__cxx11::basic_string<char>; _Compare = std::less<std::__cxx11::basic_string<char> >; _Alloc = std::allocator<std::__cxx11::basic_string<char> >; iterator = std::_Rb_tree<std::__cxx11::basic_string<char>, std::__cxx11::basic_string<char>, std::_Identity<std::__cxx11::basic_string<char> >, std::less<std::__cxx11::basic_string<char> >, std::allocator<std::__cxx11::basic_string<char> > >::const_iterator; const_iterator = std::_Rb_tree<std::__cxx11::basic_string<char>, std::__cxx11::basic_string<char>, std::_Identity<std::__cxx11::basic_string<char> >, std::less<std::__cxx11::basic_string<char> >, std::allocator<std::__cxx11::basic_string<char> > >::const_iterator; value_type = std::__cxx11::basic_string<char>]'
  548 |       insert(const_iterator __position, const value_type& __x)
      |       ^~~~~~
/usr/include/c++/14/bits/stl_set.h:548:7: note:   candidate expects 2 arguments, 1 provided
/usr/include/c++/14/bits/stl_set.h:553:7: note: candidate: 'std::set<_Key, _Compare, _Alloc>::iterator std::set<_Key, _Compare, _Alloc>::insert(const_iterator, value_type&&) [with _Key = std::__cxx11::basic_string<char>; _Compare = std::less<std::__cxx11::basic_string<char> >; _Alloc = std::allocator<std::__cxx11::basic_string<char> >; iterator = std::_Rb_tree<std::__cxx11::basic_string<char>, std::__cxx11::basic_string<char>, std::_Identity<std::__cxx11::basic_string<char> >, std::less<std::__cxx11::basic_string<char> >, std::allocator<std::__cxx11::basic_string<char> > >::const_iterator; const_iterator = std::_Rb_tree<std::__cxx11::basic_string<char>, std::__cxx11::basic_string<char>, std::_Identity<std::__cxx11::basic_string<char> >, std::less<std::__cxx11::basic_string<char> >, std::allocator<std::__cxx11::basic_string<char> > >::const_iterator; value_type = std::__cxx11::basic_string<char>]'
  553 |       insert(const_iterator __position, value_type&& __x)
      |       ^~~~~~
/usr/include/c++/14/bits/stl_set.h:553:7: note:   candidate expects 2 arguments, 1 provided
/usr/include/c++/14/bits/stl_set.h:580:7: note: candidate: 'void std::set<_Key, _Compare, _Alloc>::insert(std::initializer_list<_Tp>) [with _Key = std::__cxx11::basic_string<char>; _Compare = std::less<std::__cxx11::basic_string<char> >; _Alloc = std::allocator<std::__cxx11::basic_string<char> >]'
  580 |       insert(initializer_list<value_type> __l)
      |       ^~~~~~
/usr/include/c++/14/bits/stl_set.h:580:43: note:   no known conversion for argument 1 from 'char' to 'std::initializer_list<std::__cxx11::basic_string<char> >'
  580 |       insert(initializer_list<value_type> __l)
      |              ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~
/usr/include/c++/14/bits/stl_set.h:600:7: note: candidate: 'std::set<_Key, _Compare, _Alloc>::insert_return_type std::set<_Key, _Compare, _Alloc>::insert(node_type&&) [with _Key = std::__cxx11::basic_string<char>; _Compare = std::less<std::__cxx11::basic_string<char> >; _Alloc = std::allocator<std::__cxx11::basic_string<char> >; insert_return_type = std::_Rb_tree<std::__cxx11::basic_string<char>, std::__cxx11::basic_string<char>, std::_Identity<std::__cxx11::basic_string<char> >, std::less<std::__cxx11::basic_string<char> >, std::allocator<std::__cxx11::basic_string<char> > >::insert_return_type; node_type = std::_Rb_tree<std::__cxx11::basic_string<char>, std::__cxx11::basic_string<char>, std::_Identity<std::__cxx11::basic_string<char> >, std::less<std::__cxx11::basic_string<char> >, std::allocator<std::__cxx11::basic_string<char> > >::node_type]'
  600 |       insert(node_type&& __nh)
      |       ^~~~~~
/usr/include/c++/14/bits/stl_set.h:600:26: note:   no known conversion for argument 1 from 'char' to 'std::set<std::__cxx11::basic_string<char> >::node_type&&' {aka 'std::_Rb_tree<std::__cxx11::basic_string<char>, std::__cxx11::basic_string<char>, std::_Identity<std::__cxx11::basic_string<char> >, std::less<std::__cxx11::basic_string<char> >, std::allocator<std::__cxx11::basic_string<char> > >::node_type&&'}
  600 |       insert(node_type&& __nh)
      |              ~~~~~~~~~~~~^~~~
/usr/include/c++/14/bits/stl_set.h:605:7: note: candidate: 'std::set<_Key, _Compare, _Alloc>::iterator std::set<_Key, _Compare, _Alloc>::insert(const_iterator, node_type&&) [with _Key = std::__cxx11::basic_string<char>; _Compare = std::less<std::__cxx11::basic_string<char> >; _Alloc = std::allocator<std::__cxx11::basic_string<char> >; iterator = std::_Rb_tree<std::__cxx11::basic_string<char>, std::__cxx11::basic_string<char>, std::_Identity<std::__cxx11::basic_string<char> >, std::less<std::__cxx11::basic_string<char> >, std::allocator<std::__cxx11::basic_string<char> > >::const_iterator; const_iterator = std::_Rb_tree<std::__cxx11::basic_string<char>, std::__cxx11::basic_string<char>, std::_Identity<std::__cxx11::basic_string<char> >, std::less<std::__cxx11::basic_string<char> >, std::allocator<std::__cxx11::basic_string<char> > >::const_iterator; node_type = std::_Rb_tree<std::__cxx11::basic_string<char>, std::__cxx11::basic_string<char>, std::_Identity<std::__cxx11::basic_string<char> >, std::less<std::__cxx11::basic_string<char> >, std::allocator<std::__cxx11::basic_string<char> > >::node_type]'
  605 |       insert(const_iterator __hint, node_type&& __nh)
      |       ^~~~~~
/usr/include/c++/14/bits/stl_set.h:605:7: note:   candidate expects 2 arguments, 1 provided