Why does this not work?

Question

https://code.sololearn.com/cTLx82hln7zs/?ref=app

Answer 1

First of all you should describe what are you trying to accomplish.
It says 'Substring not found' at line 25, but I don't know what your goal is......

Answer 2

It's a game called Cows and bulls.
en.m.wikipedia.org/wiki/Bulls_and_Cows