@Ivan_Guoh, from your first post I understand the following. Your Roon Core is running on your Synology DS220+ running DSM7 and you want to write Roon Backups to a your QNAP TS-509. You are trying to create a share to your QNAP within Roon.
If that’s the situation: it’s not possible. In DSM7 within a package you no longer can create an SMB share to an other device.
Try the following. In DSM go to File Station. Within File Station mount the remote folder on your QNAP to your Backup folder on the DS220+ (Tools->Mount Remote Folder). Furthermore be sure that the internal user “roon” on your DS220+ has read/write permission on that Backup folder. Now in Roon you can create a backup to the Backup folder on the DS220+ (which by the mount in fact is a folder on your QNAP).
I have the same kind of setup. To be sure I placed my backup folder in the Roon share on the DS220+. In that share I have to subfolders: the RoonOnNAS folder and a Backup folder.
I hope this helps you further.