If your goal here is to have an FDW connection to another server ONLY on the replica, or different than the SERVER definition on the primary, I don't believe that is possible based on the error that you are getting. Whether you use a replica as a read-only or not, PG always considers a replica as a potential failover target, so that sort of data cannot be different within the replication cluster.
Once you promote that replica to a primary to allow writes, you can only turn it back into a replica again by re-syncing it with the target primary which would undo any changes you did to it after promotion.
Off the top of my head, I'm not sure if FDW connections created on the primary are actually usable on the replica as well. Would have to test and see. If so, and if you wanted another FDW connection to use on your replica, just create it on the primary and then only actually use it on the replica.
Keith
Any other option.
Bcz if I promote can I rollback again.
Regards
Rohit Sachdeva
You can promote a Read Replica via AWS CLI or AWS Web Console.
Reference:
aws rds promote-read-replica \ --db-instance-identifier test-instance-repl
Which breaks replication. You can't "demote".