raspiBackup until now supported RaspbianOS only. Raspberries 4B with 8GB and Raspberries 400 allow to install Ubuntu as operating system and it's heavily used. That's why raspiBackup will support Ubuntu Desktop and Ubuntu Server in next release.
I fortunately was able to buy a 8GB Raspi from the donations I got for raspiBackup to add and test the support for Ubuntu.
Release 0.6.9 was published today (11/19/2023) and now raspiBackup supports also Ubuntu.