That's weird, I haven't had such issue. Here's what I did (from the top of my head):

- Download the Appimage installer
- Install fastboot (in the package "android-tools" on my distribution)
- chmod +x the Appimage file, execute it
- In the installer, say yes when prompted for the first time to set udev rules; if not prompted, it's in one of the menus
- Reboot my Pro1 in bootloader mode (Vol Down + Power), plug it to USB
- In the installer, select manually the device I wanted (but automatic detection worked too if I remember correctly, it just took a little bit longer before the device was detected)
- Follow instructions: there are several steps where the installer asks you to reboot in bootloader (already done) and then press a button on the installer to continue, then boot into recovery mode and confirm, something like that

It worked well for me and I did it three times. The only time it didn't was because I had fastboot installed from snap instead of regular package manager, so it was not in my $PATH and udev rules were not set.

Also, I flashed from Sailfish but I recommend you flash from an updated Android reset package instead (the last one is from August 2020, see link in my TWRP guide) because there are some partitions that are not written by the installer and that instead reuse what was written when flashing Android in the first place. It would work still, but some features would be missing depending on how old is the image your flashed into your vendor partition. Normally, it should be enough to just flash vendor.img without reflashing Android (no data loss), this is what I did and I can confirm it fixed the missing features I observed previously. However, I am still having an issue with the fingerprints that looks very similar to one I once had in Sailfish, which suggests again a partition that is not overwritten during the flash and instead reuses what Android flashing procedure wrote in the first place, so maybe there is something else I should flash. That's why, if you're not concerned about data loss, you should avoid these potential issues if you flash Ubuntu onto a recent Android factory package.