I have trouble with the mousescroll wheel

EDIT: So I kinda know what the problem is now. I just dont know how to fix it. Made a film because its harder to explain by words in a language im not born with.

https://ufile.io/8uyt3

go to the input section and make sure there is an axis called Mouse Scrollwheel, if there is one please make sure to use the exact same name, (Probably you are missing a cap ScrollWheel)

Example:
public void Update()
{
CheckWeaponSwitch();
}

public void CheckWeaponSwitch()
{
    float mousewheel = Input.GetAxis("mousewheelname");

    if(mousewheel > 0)
    {
        //do something
    }

     if(mousewheel < 0)
    {
        //do something else
    }
}