Why this code snippet for making the window opacity into 255 is not working - Error in AutoHotKey code?

Use the explicit key name:

^!0:: ; top-row 0
^!Numpad0:: ; numpad 0 (optional, if you want both)
    opacity := 255
    WinSet, Transparent, Off, A  ; better than forcing 255
return

What you were given is good. Based on your opinion, I revised my original code.

SendMode Input
opacity := 255
^!Up::
    opacity += 15
    if (opacity >= 255) {
        opacity := 255
        WinSet, Transparent, Off, A
    } else {
        WinSet, Transparent, %opacity%, A
    }
return
^!Down::
    opacity -= 15
    if (opacity < 50)
        opacity := 50
    WinSet, Transparent, %opacity%, A
return
^!0::        ; top-row 0
^!Numpad0::  ; numpad 0
    opacity := 255
    WinSet, Transparent, Off, A
return

Now it works well.