The plugin is activated based on the purchase code verification, which is done directly on the Envato server.
It is possible that the Envato API is down, if that is the case, try to check again a little bit later.
If it’s still not working, please check on your server/website if the cURL option is enabled in your PHP settings.
Even with cURL enabled, based on security settings your server may refuse to send or receive data from the Envato server.
cURL is the only professional way that two servers connect to get data from one place to another.