Perhaps you have 2 versions of postgresql running…
Perhaps you can try first just try install the package manually before:
sudo apt install postgresql-13-postgis-3
I have got a similar issue when upgrading yunohost. I have to delete all postgres package and files before.
But be carefull, do a pgdump_all before if you have other apps working with pgsql. You need read topics about this matter different for each app then.
When looking for your problem, I found posts where they explain the problem related to old configuration of postgresql still in the congig files: