Help with placing a function please
As Juraj already explained, you cannot use Serial.read() outside a function.

What you probably want is to call it within the loop, or a function called from loop (via other functions).
Than you can call also turnMotor from Loop or a function called from loop. The value (int serial_input) probably doesn't have to be global anymore, but that depends on the time where you assign the value and where you call turnMotor.