PG_DUMP on Replica DB server Error
Parallel pg_dump against a replica is not supported cleanly until v10. 

You can either drop the -j, run the dump against the master rather than the replica, or you can use --no-synchronized-snapshots which will make your backup be potentially inconsistent.

Pausing the recovery might cure the consistency problem with use of --no-synchronized-snapshots, but the pause must be run on the replica, not on the master, which is what you seem to doing.