If you've added your Red 4Pre/8Pre/8Line/16Line to a tab in RedNet Control previously then you'll see the message "Device Not Detected" in that tab whenever the Red in question is switched off/disconnected from the computer:
If you see this message when the Red is connected to the computer via Thunderbolt then there are a few things to check:
Go to Apple Logo > About This Mac > System Report > Hardware > Thunderbolt. You should see the Red listed there:
If your Red does not show here then that would indicate that it's not being recognised by the computer on a hardware level. Please check that the Red is connected to the computer correctly via Thunderbolt and that your cables (and your Thunderbolt 3 to Thunderbolt 2 adapter, if applicable) are not faulty.
If the hardware is shown here, but RedNet Control still states shows 'Device Not Detected' then please ensure that you've installed the latest version of RedNet Control from the Downloads section of our website (or from your Focusrite.com account).
If you're running macOS High Sierra (10.13) or later you may also need to approve the driver installation manually before it will load correctly, please see this article.