Buggy python submission (verdict: Time Limit Exceeded):
import sys
sys.setrecursionlimit(10**6)
from math import floor,ceil,sqrt,factorial,log
from heapq import heappop, heappush, heappushpop
from collections import Counter,defaultdict,deque
from itertools import accumulate,permutations,combinations,product,combinations_with_replacement
from bisect import bisect_left,bisect_right
from copy import deepcopy
from operator import itemgetter
from fractions import gcd
mod = 10 ** 9 + 7
 
#整数input
def ii(): return int(sys.stdin.readline().rstrip()) #int(input())
def mii(): return map(int,sys.stdin.readline().rstrip().split())
def limii(): return list(mii()) #list(map(int,input().split()))
def lin(n:int): return [ii() for _ in range(n)]
def llint(n: int): return [limii() for _ in range(n)]
#文字列input
def ss(): return sys.stdin.readline().rstrip() #input()
def mss(): return sys.stdin.readline().rstrip()
def limss(): return list(mss()) #list(input().split())
def lst(n:int): return [ss() for _ in range(n)]
def llstr(n: int): return [limss() for _ in range(n)]

s=ss()
t=ss()
dp = [[[0,"0"] for i in range(len(s)+1)] for j in range(len(t)+1)] #0で初期化

for i in range(len(t)):
    for j in range(len(s)):
        if s[j] ==t[i]:
            dp[i+1][j+1] = [dp[i][j][0] + 1,dp[i][j][1] +s[j]]

        else:
            if dp[i+1][j][0] >=  dp[i][j+1][0]:
                dp[i+1][j+1] = [dp[i+1][j][0],dp[i+1][j][1]]
            else:
                dp[i+1][j+1] = [dp[i][j+1][0], dp[i][j+1][1]]

print(dp[-1][-1][1][1:])

Fixed submission:
import sys
sys.setrecursionlimit(10**6)
from math import floor,ceil,sqrt,factorial,log
from heapq import heappop, heappush, heappushpop
from collections import Counter,defaultdict,deque
from itertools import accumulate,permutations,combinations,product,combinations_with_replacement
from bisect import bisect_left,bisect_right
from copy import deepcopy
from operator import itemgetter
from fractions import gcd
mod = 10 ** 9 + 7
 
#整数input
def ii(): return int(sys.stdin.readline().rstrip()) #int(input())
def mii(): return map(int,sys.stdin.readline().rstrip().split())
def limii(): return list(mii()) #list(map(int,input().split()))
def lin(n:int): return [ii() for _ in range(n)]
def llint(n: int): return [limii() for _ in range(n)]
#文字列input
def ss(): return sys.stdin.readline().rstrip() #input()
def mss(): return sys.stdin.readline().rstrip()
def limss(): return list(mss()) #list(input().split())
def lst(n:int): return [ss() for _ in range(n)]
def llstr(n: int): return [limss() for _ in range(n)]

s=ss()
t=ss()
def lcs(str1, str2):
    '''文字列str1, str2の最長共通部分列(Lowest Common Subsequence, LCS)を求める
    計算量O(len(str1)*len(str2))  
    '''
    dp = [[0]*(len(str2) + 1) for _ in range(len(str1) + 1)]
    for i in range(len(str1)):
        for j in range(len(str2)):
            if str1[i] == str2[j]:
                dp[i+1][j+1] = max(dp[i+1][j+1], dp[i][j] + 1)
            else:
                dp[i+1][j+1] = max(dp[i][j+1], dp[i+1][j])
 
    '''復元を実行
    最長の長さを求めるなら return dp[len(str1)][len(str2)] をするとよい
    '''
    res = ''
    i, j = len(str1), len(str2)
    while i > 0 and j > 0:
        if dp[i][j] == dp[i-1][j]:
            i -= 1
        elif dp[i][j] == dp[i][j-1]:
            j -= 1
        else:
            res = str1[i-1] + res
            i -= 1
            j -= 1
    return res
print(lcs(s,t))

Buggy python submission (verdict: Time Limit Exceeded):
import sys
sys.setrecursionlimit(10**6)
from math import floor,ceil,sqrt,factorial,log
from heapq import heappop, heappush, heappushpop
from collections import Counter,defaultdict,deque
from itertools import accumulate,permutations,combinations,product,combinations_with_replacement
from bisect import bisect_left,bisect_right
from copy import deepcopy
from operator import itemgetter
from fractions import gcd
mod = 10 ** 9 + 7
 
#整数input
def ii(): return int(sys.stdin.readline().rstrip()) #int(input())
def mii(): return map(int,sys.stdin.readline().rstrip().split())
def limii(): return list(mii()) #list(map(int,input().split()))
def lin(n:int): return [ii() for _ in range(n)]
def llint(n: int): return [limii() for _ in range(n)]
#文字列input
def ss(): return sys.stdin.readline().rstrip() #input()
def mss(): return sys.stdin.readline().rstrip()
def limss(): return list(mss()) #list(input().split())
def lst(n:int): return [ss() for _ in range(n)]
def llstr(n: int): return [limss() for _ in range(n)]

s=ss()
t=ss()
dp = [[[0,"0"] for i in range(len(s)+1)] for j in range(len(t)+1)] #0で初期化

for i in range(len(t)):
    for j in range(len(s)):
        if s[j] ==t[i]:
            dp[i+1][j+1][0] = dp[i][j][0] + 1
            dp[i+1][j+1][1] = dp[i][j][1] +s[j]
        else:
            dp[i+1][j+1][0] = max(dp[i+1][j][0], dp[i][j+1][0])
            if max(dp[i+1][j][0], dp[i][j+1][0]) ==dp[i+1][j][0]:
                dp[i+1][j+1][1] = dp[i+1][j][1]
            else:
                dp[i+1][j+1][1] = dp[i][j+1][1]
print(dp[-1][-1][1][1:])

Fixed submission:
import sys
sys.setrecursionlimit(10**6)
from math import floor,ceil,sqrt,factorial,log
from heapq import heappop, heappush, heappushpop
from collections import Counter,defaultdict,deque
from itertools import accumulate,permutations,combinations,product,combinations_with_replacement
from bisect import bisect_left,bisect_right
from copy import deepcopy
from operator import itemgetter
from fractions import gcd
mod = 10 ** 9 + 7
 
#整数input
def ii(): return int(sys.stdin.readline().rstrip()) #int(input())
def mii(): return map(int,sys.stdin.readline().rstrip().split())
def limii(): return list(mii()) #list(map(int,input().split()))
def lin(n:int): return [ii() for _ in range(n)]
def llint(n: int): return [limii() for _ in range(n)]
#文字列input
def ss(): return sys.stdin.readline().rstrip() #input()
def mss(): return sys.stdin.readline().rstrip()
def limss(): return list(mss()) #list(input().split())
def lst(n:int): return [ss() for _ in range(n)]
def llstr(n: int): return [limss() for _ in range(n)]

s=ss()
t=ss()
def lcs(str1, str2):
    '''文字列str1, str2の最長共通部分列(Lowest Common Subsequence, LCS)を求める
    計算量O(len(str1)*len(str2))  
    '''
    dp = [[0]*(len(str2) + 1) for _ in range(len(str1) + 1)]
    for i in range(len(str1)):
        for j in range(len(str2)):
            if str1[i] == str2[j]:
                dp[i+1][j+1] = max(dp[i+1][j+1], dp[i][j] + 1)
            else:
                dp[i+1][j+1] = max(dp[i][j+1], dp[i+1][j])
 
    '''復元を実行
    最長の長さを求めるなら return dp[len(str1)][len(str2)] をするとよい
    '''
    res = ''
    i, j = len(str1), len(str2)
    while i > 0 and j > 0:
        if dp[i][j] == dp[i-1][j]:
            i -= 1
        elif dp[i][j] == dp[i][j-1]:
            j -= 1
        else:
            res = str1[i-1] + res
            i -= 1
            j -= 1
    return res
print(lcs(s,t))

Buggy python submission (verdict: Runtime Error):
import sys
sys.setrecursionlimit(10**6)
from math import floor,ceil,sqrt,factorial,log
from heapq import heappop, heappush, heappushpop
from collections import Counter,defaultdict,deque
from itertools import accumulate,permutations,combinations,product,combinations_with_replacement
from bisect import bisect_left,bisect_right
from copy import deepcopy
from operator import itemgetter
from fractions import gcd
mod = 10 ** 9 + 7
 
#整数input
def ii(): return int(sys.stdin.readline().rstrip()) #int(input())
def mii(): return map(int,sys.stdin.readline().rstrip().split())
def limii(): return list(mii()) #list(map(int,input().split()))
def lin(n:int): return [ii() for _ in range(n)]
def llint(n: int): return [limii() for _ in range(n)]
#文字列input
def ss(): return sys.stdin.readline().rstrip() #input()
def mss(): return sys.stdin.readline().rstrip()
def limss(): return list(mss()) #list(input().split())
def lst(n:int): return [ss() for _ in range(n)]
def llstr(n: int): return [limss() for _ in range(n)]

s=ss()
t=ss()
dp = [[0 for i in range(len(s)+1)] for j in range(len(t)+1)] #0で初期化

for i in range(len(s)):
    for j in range(len(t)):
        if s[j] ==t[i]:
            dp[i+1][j+1] = dp[i][j] + 1
        else:
            dp[i+1][j+1] = max(dp[i+1][j], dp[i][j+1])

print(dp[-1][-1])

Fixed submission:
import sys
sys.setrecursionlimit(10**6)
from math import floor,ceil,sqrt,factorial,log
from heapq import heappop, heappush, heappushpop
from collections import Counter,defaultdict,deque
from itertools import accumulate,permutations,combinations,product,combinations_with_replacement
from bisect import bisect_left,bisect_right
from copy import deepcopy
from operator import itemgetter
from fractions import gcd
mod = 10 ** 9 + 7
 
#整数input
def ii(): return int(sys.stdin.readline().rstrip()) #int(input())
def mii(): return map(int,sys.stdin.readline().rstrip().split())
def limii(): return list(mii()) #list(map(int,input().split()))
def lin(n:int): return [ii() for _ in range(n)]
def llint(n: int): return [limii() for _ in range(n)]
#文字列input
def ss(): return sys.stdin.readline().rstrip() #input()
def mss(): return sys.stdin.readline().rstrip()
def limss(): return list(mss()) #list(input().split())
def lst(n:int): return [ss() for _ in range(n)]
def llstr(n: int): return [limss() for _ in range(n)]

s=ss()
t=ss()
def lcs(str1, str2):
    '''文字列str1, str2の最長共通部分列(Lowest Common Subsequence, LCS)を求める
    計算量O(len(str1)*len(str2))  
    '''
    dp = [[0]*(len(str2) + 1) for _ in range(len(str1) + 1)]
    for i in range(len(str1)):
        for j in range(len(str2)):
            if str1[i] == str2[j]:
                dp[i+1][j+1] = max(dp[i+1][j+1], dp[i][j] + 1)
            else:
                dp[i+1][j+1] = max(dp[i][j+1], dp[i+1][j])
 
    '''復元を実行
    最長の長さを求めるなら return dp[len(str1)][len(str2)] をするとよい
    '''
    res = ''
    i, j = len(str1), len(str2)
    while i > 0 and j > 0:
        if dp[i][j] == dp[i-1][j]:
            i -= 1
        elif dp[i][j] == dp[i][j-1]:
            j -= 1
        else:
            res = str1[i-1] + res
            i -= 1
            j -= 1
    return res
print(lcs(s,t))