The clue is in the error message. It is complaining that it expects a type “bool” value but it’s got a type “float” value and tt won’t change the type without explicit instructions to do so.
Input.GetAxis() doesn’t return a bool value, it has a return type of float. Try feeding the output of Input.GetAxis() into a comparison operator such as ==0 or <0.1.
if (Input.GetAxis("Mouse ScrollWheel") != 0){
// Do something.
}