Explain the reason why this won't execute

Question

https://code.sololearn.com/c7N0up7MfbMX/?ref=app

Answer 1

You were really using the old versions of C before Sololearn, clrscr() just isn't gonna work here.

and use int main(), though it'll still work

Answer 2

Line 10 replace function() with int function().

Then, what is clrscr()? It isn't defined.