Good afternoon.  Are you attempting to associate the new copy to a Storage Pool already used in this storage policy?  You can only use a storage pool once per storage policy.  If you have already used a storage pool in this storage policy, please select another and retry to add the copy.

no i want to assign another media agent with other storage library here to move backups to new library instaed of already added.

@AlphaFawad , it sounds like you want to move the copy to a new library, is that correct?  Do you want all of the existing jobs to move as well?

you can create a Synchronous Aux Copy to the new library, let it complete, then promote that new copy to the Primary.

Nice and clean.