(not Lightroom CC). Go to File > Plug-in Manager . Confirm Negative Lab Pro is listed and enabled.
It is good practice to confirm that your license is permanently active and not running in trial mode. negative lab pro license key install
If you have tried everything and still can't get your license to work, reach out for help. (not Lightroom CC)
Here is a step-by-step guide on how to install your Negative Lab Pro license key correctly. negative lab pro license key install