Where is this “not compatible” message showing up? Is it in the iOS Viewer app?
Can you maybe send us the screenshot of this error message?
Can you please also tell us the info of your iPad device (e.g. model, iOS version)?
What is the current spacedesk Viewer version installed on your iPad?
Apple officially does not support iOS version 9.x/10.x as Deployment Target anymore since the latest Xcode IDE update.
All the other apps in the Apple App Store have also lost their compatibility to iOS 9.x/10.x since the latest updates.
Therefore we cannot support these iOS versions anymore.