[postgresql] PostgreSQL: Why psql can't connect to server?

In my case I had this error, /var/run/postgresql/.s.PGSQL.5433 (note, one number up from the file it was looking for, .s.PGSQL.5432) was present. Tried the instructions at the top of this page but nothing worked.

Turns out there was an old directory for PostGreSQL 12 config files in /etc/postgresql/12, which I deleted, which solved the issue.