There's a minor issue that causes Cubase to blacklist Auto-Tune Pro 9.1 when it's first scanned by Cubase's plug-in manager. Fortunately there's a simple workaround, which is outlined below.
1. You will likely see this pop-up in Cubase the first time after installing and launching Auto-Tune Pro 9.1:
2. Click OK, go to the Studio menu, and select VST Plug-in Manager.
3. Select the "Auto-Tune.vst3" plug-in from the list and click on the Reactivate button.
4. Select Reactivate again in the following pop-up screen to remove Auto-Tune Pro from Cubase's blacklist.
At this point you should be able to use Auto-Tune Pro 9.1 in Cubase.
We are working hard to resolve this issue in the next maintenance update. Thank you for your patience.