If you have a valid database backup of v13 already, then install Debian 10, then ERPNext, then restore your database and do a “bench migrate” to make sure the database is good.
Instructions for doing the install on Debian 10 can be found here:
There is a growing number of people on the forum here (including myself) that have given up on Ubuntu. The support packages for it change frequently and there are too many mutations of it spread out over the VPS providers that it is nearly impossible to get a single set of instructions.
Debian is much slower to change and has a longer stability that the other OS systems. When I wrote the install instructions for it, I did it within a week of the Ubuntu instructions. After revisiting the Ubuntu instruction for the 5th time to adjust for changes, I finally gave up and move all of my servers to Debian 10.
I used to be like you (a Ubuntu loyalist) but had to give up on that. I have not had any trouble with the Debian servers.
Hope this helps.
BKM