First, thanks to everyone who is trying to help! I followed your instructions Leigh, and I got the same result as before (screenshot attached). I'm running on an older iMac and have been advised by our Apple repair service not to attempt to upgrade. The program itself seems (so far) to run just fine on my computer - it's just that I am unable to register.
I also purchased the app from the Apple app store and am unable to register. I'm running High Sierra 10.13.6. There does not seem to be a "Run as Administrator" option on the Mac.