Hi,
I had a similar trouble as yours : unable to install UT => https://github.com/ubports/ubports-installer/issues/1070
In fact, I previously installed an unofficial rom of LineageOS => https://forum.xda-developers.com/google-nexus-5/orig-development/rom-lineageos-16-0-nexus-5-t3921162
To do it, I needed to resize the partition => https://forum.xda-developers.com/showpost.php?p=79928889&postcount=190
So, I have done a revert partitionning and everything went well, I was able to install UT.