When an active duty member is being separated for non-selection, are they unable to join Guard or Reserve units?

I do believe you can join the Guard or Reserve. Your Career Counselor will be your first stop IOT get definitive answers

I changed the wording of your question to match the text of your post (you asked "are you able" in your topic statement and "is one unable" in the body ... so an answer would be confusing based on what it was applied to).

Specific to your question - it depends - are you a regular Air Force commission or a Reserve Air Force commision on active duty? Is your MOS considered a critical skill in the Air Force (any COMPO)? Etc. Generally, the answer is "yes you can join the RC" but depending on your specific situation many things could impact you.

As others have pointed out - talk to a recruiter for the service component you want to join.

You could also try rebranding to the Army. We have a shortage of Pharmacists