Can't activate serial key - legacy activation method (the serial is not valid for this product)

If you're trying to activate an old unsupported license using the legacy serial key method and getting the error 'The serial number (xxx) is not valid for this product', the most likely cause is that you're using the license ID and not the serial key.

Serial keys look like this: 123-456-123456-1234.

With user account licensing, you no longer need your serial key to activate your license if you have a supported license. You can activate by logging in with your Redgate ID and this ensures that you never run out of activations. On the toolbar of the tool, click on Help > Manage my license. 

If you're activating an old license on a machine without an internet connection you will need your serial key. As they're no longer prominently displayed in the customer portal, click on the Licenses tab and switch to the old licensing view:

Click on the tool you want, and you'll see the Serial Numbers tab: