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.