What is the problem with level 0 of the c language coding test?

I want to find out one by one while solving the programmer's coding test problem by taking a log point in the visual studio, but I tried solving the programmer's problem without the main door using the main door, where is the problem? This is a specific character replacement problem! There is no problem with the solution function by matching the problem!

char* solution(const char* my_string, const char* letter);

int main(void) {

char str[8] = "hello";
char letter = 'h';

solution(str, letter);
printf(str);

return 0;

}

char* solution(const char* my_string, const char* letter) {

int len = strlen(my_string);
int j = 0;
char* answer = (char*)malloc(len - 1);

for (int i = 0; i < len; i++)
{
    if (my_string[i] != letter[0])
    {
        answer[j] = my_string[i];
        j++;
    }
}
answer[j] = '\0';

return answer;

}