They are in /var/discourse/shared/standalone/backups/default
If you follow the rsync instructions I linked earlier, you’ll get them.
It crashed or the server rebooted or something things happen.
The database is “migrated” from one set of tables (tables get added and changed) to another on most upgrades.
You might try to stop the container and delete that lock file
And look in PG_VERSION to see what version you have, since I think you tried changing the template.