Haven't done a session yet but think I fixed my problem.
Apparently some recent Mac operating systems, like Mojave, require that you "allow" the software you load or it won't recognize it. I am using a Focusrite Scarlet 2i2 3rd Gen. Focusrite has good tech support on their site (JamKazam not so much!) with easy to understand answers to common questions/problems.
Here is an abbreviated version of what I found (after a week of pulling out whats left of my hair!):
"With the release of macOS 10.14 (Mojave), Apple has released a new security feature which requires the user to manually "Allow" apps to access the microphone. When opening an audio app for the first time you will be presented with the below message.
In order to input audio into your apps using your Focusrite interface, you will be required to press "OK".
If you have clicked "Don't Allow" for any reason, you will need to open the System Preferences and go to Security and Privacy to manually allow the app to access audio input. Once you're in Security and Privacy, click on the Privacy tab, then select Microphone on the left-hand side and select the apps you wish to access the microphone."
I followed these instructions and "allowed" Focusrite and JamKazam (for good measure!) and did a "Solo Session' as a test. Seems to have solved the problem.
Thanks for all the suggestions here. Hope this bit helps the next TBer.