My MIDI Keys (or none of my keys in Cubase) are responding in Stream Deck

There are rare occasions where the keys do not respond at all and it is due to a conflict with the Elgato Control Center.

Go to the Stream Deck software and right click on any of the Control Center tools and select Uninstall.

See if the keys are now working. Then you can go back to the Elgato Marketplace and reinstall the Control Center.