Arm for record disappears

I use to be able to begin a new multi-track session and select arm for record.

Now, when I would open a new multi-track, the "R" was grayed out. I can add a new track to record into, but if I save and close the session to come back to later, I can no longer arm the track.

I'm on a Mac. I've checked Audition > Preferences > Audio Hardware to make sure the microphone is selected. And still I cannot arm the track to record.

What am I missing?

Thank you.