Google Pixel 3a - Installation Problem

Hello.
I received my refurbished Google Pixel 3a yesterday to install Ubuntu Touch on it. I don’t want to use a Google or Apple OS anymore. So far, I’ve read some guides, unlocked the bootloader, and enabled USB debugging. I installed UBports on my Lenovo laptop (operating system on the laptop: Ubuntu). Everything went well. But: the boot process, after the “successfully completed installation“, always ends at the recovery screen. I’ve also tried using fastboot and searched for guides, but I always end up at the UBports recovery screen. I wanted to ask if you might have some advice from experience on how to figure out where the problem is. When I enter fastboot mode now, I see that the device is unlocked, Boot Slot A, and at the bottom it says: Enter reason: combo key.

I’d really appreciate any help.

Best.