I upgraded from 12.7 to 2019.5 without getting properly prepared. I wanted to be able to work with OSX 10.14.3 which I also upgraded to before thinking.
Took me a full day of pulling my hair out trying to get things to work. After installation, I could not add tracks or insert or remove plugins, regardless of New or Old sessions. Here is what worked for me. By the way, my rig is a 2016 MacBook Pro 2.7 - i7 with 16 gig of ram.
I deleted all of the preference, workspace and wavecache files as well as database files. Avid explains this fairly well, though it shows two files to delete that I couldn't find. Anyway, thereafter I deleted the installation of ProTools 2019.5 by moving the app to the trash bin. Did this twice to find it still didn't work.
So I repeated the process again, but this time I also deleted every folder with the name AVID or PROTOOLS. Backed up the PlugIns first by the way. After this delete process I reinstalled PT 2019.5 for like the 4th time and it worked. Seems it was the deletion of the FOLDERS left behind that made the difference.
I swear I'm never updating anything again! Anyway, hope this helps anyone in the same boat.