This keep happening with some users over the time. Most likely some issues at the Apple's servers end.
In case this issue is reported when you are fully confident that your device has the capability to run the said app, the most likely explanation is a temporary glitch from Apple's end.
The recommended approach here is to wait for some time and try again.
The iCloud Drive app in iOS 11 and later is superseded by a new app called Files by Apple.
Files app support connecting to multiple storage sources with iCloud Drive connected by default.
iCloud Drive app is still available on the App Store for older devices running iOS 10. Files app is only available for iOS 11 and later.
Since both the (similar) apps need to remain available on the App Store for supporting iOS 10 and 11, this approach has been taken where the Files app is supported only on devices running iOS 11 and later, while iCloud Drive app is restricted from getting installed on iOS 11.