If ‘Device not found’ appears in the Airport Utility app on your Mac, it means that your AirPort Extreme, AirPort Express or Time Capsule is not showing up in the AirPort Utility app and hence, not accessible. Nevertheless, your AirPort or Time Capsule device is still working and reachable via Finder app. The Fix ‘Device not found’ in AirPort Utility for AirPort Extreme, AirPort Express and Time Capsule is done by unplugging the AirPort device and then deleting Apple ID in Back to my Mac.
