To add, I tried to install the stock image from Android -- specifically 6.0.1 (M4B30Z, Dec 2016), found here, which gave some errors and failed to fully install (though it did change the look of the bootloader.) I still was stuck with the UBports Recovery program.
So, I looked around and found the TWRP recovery program for hammerhead (Nexus 5 codename) and successfully installed that. I used it to wipe and format the drive, since all my previous attempts at installations (of either Ubuntu Touch or Android or LineageOS) failed.
Then, I tried once again to install LineageOS, by transferring the file to the phone (see code below) and then using TWRP to install it. This worked.mark@debian:~/Downloads/Nexus5/LineageOS$ adb push lineage-14.1-20190301-UNOFFICIAL-hammerhead.zip /sdcard/. lineage-14.1-20190301-UNOFFICIAL-hamme... 4.4 MB/s (329881284 bytes in 71.197s)
I must say that the UBports recovery program needs improvement. It's far inferior to the TWRP recovery program.
Anyway, my issue is solved. I can now reinstall Ubuntu Touch.  I did reinstall it. So, I'm good to go.