It is good practice to confirm that your license is permanently active and not running in trial mode.

When the interface opens for the first time, a registration prompt will appear.

Negative Lab Pro v3+ requires (or Lightroom 6 if using legacy versions). If you are using a very old version of Lightroom, the plugin will not load.

What of Lightroom Classic and Negative Lab Pro do you have installed? What specific error message or behavior are you seeing?

Find your unique license key in the confirmation email sent by Gumroad (the payment processor for Negative Lab Pro) at the time of purchase.