It looks like databases/pg_partman in ports is rather old and emits error during creation of native partition with recent pgsql.
ERROR: column "relhasoids" does not exist
Tables in postgresql 12 doesn't contain OIDS anymore.
It was fixed in 4.2.0 https://github.com/pgpartman/pg_partman/issues/286 , but the latest version now is 4.4.0, so port needs an update.
See update of databases/pg_partman