Language: C

int calculate_digit_sum (int target)
{
    // return value of this function
    int digit_sum;

    // STEP.01
    // initialize the variable to store the digit sum
    digit_sum = (int)0;

    // STEP.02
    // calculate the digit sum
    while (target > (int)0)
    {
        digit_sum += target % (int)10;
        target    /= (int)10;
    }

    // STEP.END
    // declare the return value of this function
    return digit_sum;
}

bool is_multiple (int target, int base)
{
    if (target % base == (int)0) { return true;  }
    else                         { return false; }
}

int main (void)
{
    // variables for `main`
    int int_given;
    int rtvl_scanf;

    // STEP.01
    // read out the given string
    rtvl_scanf = scanf("%d", &int_given);

    // STEP.02
    // output whether the digit sum of given integer is the multiple of the given integer or not
    if ( is_multiple( int_given, calculate_digit_sum(int_given) ) )
    { printf("%s\n", "Yes"); }
    else
    { printf("%s\n", "No"); }

    // STEP.END
    return 0;
}

Compiler output:
main.c:23:1: error: unknown type name 'bool'
   23 | bool is_multiple (int target, int base)
      | ^~~~
main.c:1:1: note: 'bool' is defined in header '<stdbool.h>'; this is probably fixable by adding '#include <stdbool.h>'
  +++ |+#include <stdbool.h>
    1 | int calculate_digit_sum (int target)
main.c: In function 'is_multiple':
main.c:25:43: error: 'true' undeclared (first use in this function)
   25 |     if (target % base == (int)0) { return true;  }
      |                                           ^~~~
main.c:25:43: note: 'true' is defined in header '<stdbool.h>'; this is probably fixable by adding '#include <stdbool.h>'
main.c:25:43: note: each undeclared identifier is reported only once for each function it appears in
main.c:26:43: error: 'false' undeclared (first use in this function)
   26 |     else                         { return false; }
      |                                           ^~~~~
main.c:26:43: note: 'false' is defined in header '<stdbool.h>'; this is probably fixable by adding '#include <stdbool.h>'
main.c: In function 'main':
main.c:37:18: error: implicit declaration of function 'scanf' [-Wimplicit-function-declaration]
   37 |     rtvl_scanf = scanf("%d", &int_given);
      |                  ^~~~~
main.c:1:1: note: include '<stdio.h>' or provide a declaration of 'scanf'
  +++ |+#include <stdio.h>
    1 | int calculate_digit_sum (int target)
main.c:37:18: warning: incompatible implicit declaration of built-in function 'scanf' [-Wbuiltin-declaration-mismatch]
   37 |     rtvl_scanf = scanf("%d", &int_given);
      |                  ^~~~~
main.c:37:18: note: include '<stdio.h>' or provide a declaration of 'scanf'
main.c:42:7: error: implicit declaration of function 'printf' [-Wimplicit-function-declaration]
   42 |     { printf("%s\n", "Yes"); }
      |       ^~~~~~
main.c:42:7: note: include '<stdio.h>' or provide a declaration of 'printf'
main.c:42:7: warning: incompatible implicit declaration of built-in function 'printf' [-Wbuiltin-declaration-mismatch]
main.c:42:7: note: include '<stdio.h>' or provide a declaration of 'printf'
main.c:44:7: warning: incompatible implicit declaration of built-in function 'printf' [-Wbuiltin-declaration-mismatch]
   44 |     { printf("%s\n", "No"); }
      |       ^~~~~~
main.c:44:7: note: include '<stdio.h>' or provide a declaration of 'printf'