Does the Adobe Tech Note linked by the first button say anything useful?
Lightroom Classic upgrades a copy of the existing catalog, so even if something goes wrong like this, the version 13 catalog should still be in that same folder, unchanged. You can use the Creative Cloud app to uninstall 13.3, then select 13.2 or earlier to be installed.
After reinstalling 13.2 and opening the version 13 catalog, it’s a good idea to choose File > Optimize Catalog in case that clears up any catalog database problems that prevented the upgrade. At that point you can just stay on 13.2 for a while, or try upgrading to 13.3 again.
When LR upgrades a catalogue, it leaves the catalogue file in the same folder and tries to create an upgraded copy, and it is the latter which will be used by 13.3.
Unless we see a slew of similar reports, I doubt we will learn why this happened or that you will be able to fix the upgrade problem yourself. Sure, you can try restarting the computer, deleting the preferences and other tricks, and sometimes it just works when you try a second time. But here I feel you need to get working with 13.2 again and let Adobe fix the upgrade problem. So save a copy of your catalogue file, contact Adobe support, and tell them about the upgrade problem. They should try it themselves on your catalogue. If there's a bug, maybe there's a bug fix release coming. Or they may say "trash your prefs", or ensure external drive X is attached, etc etc.
You can get LRc 13.2 working again with the not-upgraded catalogue. Go into the CC app, uninstall LrC 13.3, and then install LrC 13.2. This should now open your catalogue and you can carry on working.
Yes, re-installing 13.2 worked. I had backed up and optimized the catalog before upgrading to 13.3. I might give it a try again in a month. Thank you for the help.