In order to activate the plugin successfully, please verify the following items:
1) Make sure that you have empty/available URL spaces for your license in your Control Panel. This is notated by the number of "available licenses" listed in your main license area of your Control Panel.
2) Verify you are using the correct API key for the main WP-Client plugin, and not the API key for one of the WP-Client extensions. The main API key is located near the top of the Control Panel page.
If you are confident the above items are correct, you may be experiencing an issue connecting to our activation site. This could be a problem with cURL or a firewall at your server. cURL should be enabled on your server in order for WP-Client to be able to activate.
If you continue to experience issues with activation, please contact our Support Team by creating a ticket.