You may want to provide more information (I assume you are talking about Lightroom) such as:
- What version is the serial number for?
- What version are you trying to enter the serial number in?
Also you might copy/paste instead of typing to make sure something’s not being misinterpreted.
You also need to ensure - if this is a new ownership/installation of Lr - that you've bough a full licence, not an upgrade.