-23 Error: License number already used

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