I had the same issue on my SGP771 device no matter which version of UBports Installer I have tried and which settings I have chosen. After not being able to find any clue I have installed Ubuntu 22.04 on my old desktop and went through these steps to install UT without the installer:
Flashed my device with Android 7 using Flashtool.
Downloaded system.img, halium-boot.img (old) and TWRP-3.1.1-lineageos-karin-20170520.img using links from Guf's porting page.
Installed custom recovery TWRP-3.1.1-lineageos-karin-20170520.img on the device using these instructions.
Installed system.img and rootfs with halium-install + flashed halium-boot.img as described here.
I ran those two blocks of code on Guf's porting page in TWRP command line (under “Important note for UT rootfs to work”).
I am a newbie in this area and not sure if that is all correct but UT is running, except for the camera and bluetooth, I didn’t figure out yet how to enable them.