This alert may appear when you launch Reason in ReWire mode with Cubase and "Release driver when Application is in Background" has been activated in Cubase's Device Setup. It is important that Cubase doesn't release the driver when running as a ReWire host since the ReWire host is processing all audio. Here's how to configure Cubase not to release the driver:
1. Ensure that Reason is not running
2. Launch Cubase
3. Open Device Setup from the Cubase Devices menu.
4. Select VST Audio System in the left frame.
5. Deselect "Release driver when Application is in Background" checkbox in the right frame of the window.
Note that Reason 11 and later does NOT feature ReWire, since you can use Reason 11 as a Rack Plugin in a compatible host DAW.