Yes, I'm using the minimal from 20.04 but I was getting these same errors and dialog in both versions. Have an update on the progress...
There is an error around line 32/33. It dumps the user in nano but doesn't actually do anything. I navigated to the top and hit CTRL+K to remove the first line. Not sure if I was at the end of the file because I navigated there previously under the expectation it would be the last line (that's how I add it myself when doing those steps for my Ubuntu installs) but for someone not familiar with things this would be puzzling. After I removed the line, saved the file and exited nano the script resumed as expected. So far, so good...
...and it's done. I guess I do a sudo reboot and see what happens!
EDIT: Restarting got me a nice Plymouth screen for a bit and then dumped me at terminal with the message that my drive was clean. Switching to terminal one gave me login, so I went ahead and entered my credentials....and still nothing. Starting to suspect something broke but I don't know what. I'm sitting at terminal with my account logged in. Was something supposed to start Pegasus-FE? Should I type 'startx' or something else?
Yeah I know line 32/33 just dumps the user in nano where you have to remove the line manually, I will check if I can get it removed by using a command instead.
Please use Ubuntu 18.04 minimal instead, might be some changes in 20.04, I had issues with that dist too (don't remember exactly what though). Yes, it should start pegasus-fe automatically.
Btw: did you install pegasus-fe from retropie-setup script in experimental packages? I'm going to reinstall my NUC (been using LibreELEC there for 2 years so finally gonna make the switch to Ubuntu instead, might as well do it now :))
Edit 2: I've just ran the setup process without any issues on my Intel NUC, it boots straight into Pegasus-FE. Might add a section to the script where it pulls todays current version of Pegasus-FE, otherwise it's better to just use RetroPie-Setup script to download it to get the latest version.
Edit3: To enable ports, please install lr-prboom or similar package from RetroPie Setup script