I was able to get my app running on 10.15b3 by copying it from shared folder (running on Parallels)
During startup of the app check that temp folder exists and is writeable. As i was able to run my app without problems, I assume the temp folder was there.
After doing some tests with notarization I have been able to install my app via a downloaded pkg installer, but I can no longer pass the temp folder check. Even my copied app is not working.
The install was an upgraded 10.14 copy and I think I rebooted inbetween tests.
So maybe you just have to upgrade and then never reboot the computer?