"ERROR: column rel.relhasoids does not exist" with PostgreSQL 12 and PgAdmin 4.3
The column relhasoids has been dropped from the system catalog pg_class in PostgreSQL v12, because the long deprecated WITH OIDS clause has been removed from CREATE TABLE.
Obviously you are using an old version of pgAdmin 4 that has not got the message yet.
Upgrade to a newer version of pgAdmin, and the error should vanish.