Hi @sohahm !
If you are restoring the backup to another cluster you need to specify backupSource in your restore yaml definition instead of using backupName. If you are already using this and pointing the restore to correct clusterName in that case it might be some kind of a bug so please provide more info (yaml files used etc.).
You can find more info about backupSource here: Backup and restore - Percona Operator for MongoDB
Since you probably created a wrong restore object I think you will need to delete the old one with something like kubectl delete psmdb-restore <restore_name> so that operator stops trying to do faulty restore.