The possible causes for this error message:
- The serial you are attempting to activate with has reached the maximum number of installations allowed for your serial number.
- The serial number has been marked as inactive, most likely due to an upgrade or refund.
- The Nitro Computer ID has been marked as activated previously.
You will need to deactivate Nitro Pro from another computer, before you can attempt to activate again.
To do this: Open Nitro Pro (on a machine you will no longer be using) > click on the Help tab > About Nitro Pro > Deactivate