Hi
"Registering Backup and Applying Retention" means store the metadata information of the backup on the SnapCenter database and then apply the retention by deleting backups and snapshots accordingly. So, or the comunicaiton with the database has some problme or you have a large number of backups/snapshot to check if they need to be deleted.
Do you see some timeout or failure or the prcess in the end succeeds?
You can try by configuring the appropriate retention and running the PS cmdlet "Remove-Smbackup -CleanupSecondaryBackups" regularly to cleanup the metadata entries