So I use the iDrive app all the time. iDrive is trying to pull a fast one on NAS users. They now have a NAS backup service where you can use Hybrid Backup Sync to backup your NAS. But the cost to do that is EXPENSIVE. The iDrive app treats your NAS like any other Linux server and uses normal iDrive backup space.
If the iDrive app won’t install from the App Center, install it manually. To do that, download it from the App Center:
https://www.qnap.com/en-us/app-center/?os=qts&version=5.2.1~5.2.7
Looks like your NAS would be a TS-x31x from the NAS selection menu.
To manually install the app after downloading the package, go to the app center. Click the install button (second from right at the top with the little “+” sign) and select Install Manually
Then in the window that shows up, select the local copy of the iDrive package your downloaded and complete the install.
Should work just fine.