Rest assured your key is still valid. If the activation server is temporarily unavailable, your copy of Windows will automatically be activated when the service comes back online. So, please try again later.
For support, you may contact Microsoft Support Team here https://support.microsoft.com/contactus.
To assist you better, please include the details below:
1. Order ID
2. Screenshot of the pop-up/error message
3. Additional information (if any)