This is probably because you have a registration key for a previous version of NXPowerLite and are trying to use it to unlock the latest version.
If your key is for an earlier version, NXPowerLite versions 1-8 are available for existing customers to download from this support article.
Alternatively, you can upgrade to the latest version at 50% of the price of a full licence in our online store.
Any customers who have current Annual Maintenance cover should have received the latest key free of charge. If you haven't received it yet, please contact us for your new registration key.