1
Open the Settings app, then go to the activation settings section.
Click the Troubleshoot button, then select 'I changed my hardware' and in the resulting list, select the previously activated device, that is the process to move your licence to the new hardware/PC.
2
If that process is failing.
Where did you get your Windows Pro licence did you purchase a full retail Windows licence direct from Microsoft or from a 3rd party retailer, and if so, do you have the product key?