When activating SquaredUp you see the following message:
"The activation key you have entered is incompatible with this version of SquaredUp"
The activation key being used is not valid for the version of SquaredUp. For example, this error occurs when using a Community Edition (CE) activation key on SquaredUp v4. CE licences can only be used on SquaredUp v3.
For more information see How to activate your licence.