Before proceeding, make sure you have update Cubase/Nuendo and Reason to their latest respective updates.

1. In Cubase/Nuendo, go to Devices > Device Setup.
2. -Click on VST Audio System. It may show up as VST Audio Bay or vST Multitrack, depending on the version of Cubase.
3. Uncheck the box for "Release ASIO drive in background."
4. Quit Reason and Cubase.
5. Relaunch Cubase and try Reason again.