If you have a legitimate Fern Pro license key but it isn't working, try these steps:
Are you trying to during the activation process? fern pro license key
Direct access to developer assistance for troubleshooting. How to Obtain a Valid License Key If you have a legitimate Fern Pro license
Make sure the key matches the version of the software you have installed. fern pro license key
Some keys are locked to a specific computer; you may need to "deactivate" it on an old machine first.