Problem creating subscriptions on Replication Server version 15.7.1
The error message indicates you're using a WarmStandby (WS) setup, in which case there are no subscriptions required (nor supported), period.

If you're trying to use the WS pair as a source for replicating into a 3rd db, then the repdef will need to reference the WS logical pair as the primary db.

If you're trying to replicate into the WS pair from a 3rd db, then the subscription will reference the WS logical pair as the replicate db.