Hello,
Some people have had difficulty satisfying this prerequisite to installing Ubuntu Touch on the Pixel 3a XL...
I had my share...!
So, in case this helps anyone:

After flash-all.sh & adb sideload methods failed with errors... and TRWP wouldn't load (fastboot boot twrp.img) or install...
I persisted, flashing individual .img files - still not booting...
TWRP eventually came up! (fastboot boot twrp.img)

• I used TWRP to adb sideload the Google OTA Android 9 .zip (to slot a)
• Then I used adb sideload with stock recovery (to slot b)
• I used stock recovery adb sideload (to slot a)
• Then I used stock recovery "Factory Reset"
• Android 9 booted.
• UBPorts Installer ran perfectly.

What seems to have worked for me:

• Persistence
• Sideloading Android 9 OTA to BOTH slots a and b
• Factory Reset in stock recovery after all that

[I use a Linux PC; latest adb & fastboot;]
[latest version Google Stock Android 9 files]

Don't give up.
KJU

Hello.
Ubuntu Touch is running nicely on Pixel 3a XL bonito.
Many thanks for that.

In order to make a full backup and store it off-device, can I safely boot TWRP over a USB connection with:

• 'fastboot boot twrp.img'
• and only use the backup function
  ... then reboot UBTouch without installing TWRP?
  And, would the backup be TWRP-restorable?

I have faith that somebody knows!
Cheers,
KJU

However, 'Android Flash Tool' by Google makes the task of restoring stock factory Android to Pixels...
So... much... easier.
(It took me a while to find it, that's all.)

https://flash.android.com/welcome

Access is by opening that website in Chrome or Edge or another of several compatible browsers.

When it completes, it offers to lock the bootloader, but that can be skipped.
KJU