A square tile with side 5 has an area the caller wants to print. The starter already calls square(5) from main and prints the result. Define square above main so its declaration is visible at the call. Have the function return the area; leave printing to main.
int square(int side) above main to return side * side. Keep the call in main so the program prints exactly Area: 25 and a newline.The output check covers only square(5). Try an argument of 3 and look for Area: 9, then restore 5. A hard-coded return of 25 would pass the submitted case but fail this one. Keep the multiplication in square and the print call in main.
The return type is int, so return side * side; rather than printing inside the function.