Hey Rob,
Your license can be activate for 1 live version and 1 staging version of your site. For more details, please see Product Validation.
I checked your assigned sites and you have not assigned a staging domain yet. Please assign your staging domain and provided there's no connection issues, it your staging site should be validated.

You can use X in staging without validation however, you will not get the convenience of auto-updates.
Hope that helps.