Error message in codeblocks: Undefined reference to 'WinMain@16'

I want to run the following code in Codeblocks 20.03, but I get the error message: Undefined reference to 'WinMain@16'.

Code:

std::string countSheep(int number) {  std::string res;  std::string s = " sheep...";  for (int i = 1; i <= number; i++) {    res += std::to_string(i) + s;  }  return res;}

The initial problem was, that I got the error message: to_string’ is not a member of ‘stdAfter that I have updated my GCC compiler to 9.2.0. But now, I keep getting that winmain error message.

Do you have any idea, what to do?