@kuuga What OS are you using for your host PC in the installation?
If ADB cannot see the device, especially when the phone is in "fastboot" mode, with a windows host, this is probably because of missing google android drivers.
On a Linux host, the command line tools from google should just work as-is after extraction. (In my bumbling around I used both.)
Windows google-android driver: https://developer.android.com/studio/run/win-usb
This guide below was extremely helpful for me, specifically the sections "Prerequisites" and "Unlocking Bootloader" since google's developer pages have so many dead 404 links. The other sections can be ignored if using UBPorts installer.
https://xdaforums.com/t/guide-pixel-3a-sargo-unlock-bootloader-update-root-pass-safetynet.4443459/
Don't forget to downgrade your Pixel3a to Android 9 before running UBPorts installer.