Roland V-Drums/GarageBand midi HELP!!

On Garageband, there is no midi out. Therefore, your Roland is only sending midi messages to trigger the sounds in a GB software instrument. Each drum is assigned a note, and when the midi message for that note comes through, that drum plays.

GB has a capability to manually play a software instrument from the computer's keyboard, called musical typing:

Play software instruments in GarageBand on Mac - Apple Support

It looks like this:

If you select your drum track your drum software instrument will show at the top.

(If your Air has a touchbar, you can use that to play the drums, too.)

As you play the keys, different drum sounds will be heard. Find your high hat open and high hat closed and crash cymbal. determine the note and octave.

Once you know the midi note you go into the roland's settings to change it:

BTW, the GB users guide for Mac is here:

macOS app user guides: GarageBand for MacUser Guide - Apple Support

Can you play those drums with musical typing?

If so, you probably need to change the note sent for that trigger on the roland.

Note that if you switch to Garageband's Smarter Brother, Logic, you would be able to use the myriads of sounds on your V-Drums, since it can receive and send midi.