You don’t assign the Axis… There are two different types of axis inputs.
R AXIS Z and R AXIS Z+ are two different inputs. The R AXIS Z are analog controls used with the spoilers movement and position. You can’t assign this into a digital command like Extend Spoilers. But you can assign this to Spoilers Axis.
R AXIS Z+ are digital control input triggered by the specific position of the spoiler lever. You can’t assign this to a spoiler axis command but you can assign this to the Extend Spoilers command.