X_REPLICATOR_SLOT_HELD
Means
another replicator already owns this database
Typical cause
x dev --role replicator started while another process holds the database's replicator lock (a container that loses the lock stays up unready and takes over when it frees, so it never throws this) — nothing is wrong with this process, the database already has its one replicator
Fix
x dev --role replicator # rerun after the replicator already on this DATABASE_URL stops — one database has one replicator