If you have an evaluation license installed, Aruba ClearPass will give an error when you try to add a permanent licensing using "Add License".
Try this:
Administration -> Server Manager -> Licensing
Select "Applications"
Click on "Evaluation"
A new Window pops up and here you can enter the permanet license.
This is required if you upgrade ClearPass from legacy license to new-license, e.g from v6.6 to v6.8.
Legacy license is before v6.7
New-license is from v6.7 and newer.