Make sure the NUC unit is connected to a stable internet connection and activate the Windows License Key found on the underside of the Product.
Example:
A Windows License Key consists of a 25-character code used for activation.
License Key : XXXXX-XXXXX-XXXXX-XXXXX-XXXXX
Copy the Windows License Key and enter it with the following steps:
Click Start > Settings > Update & security > Activation > Change the product key.
Enter the new product key then click Next to Activate.