There might be three reasons why your add-on license cannot be activated:

1. Your cURL is outdated. You might have got this error when trying to activate your add-on license: 'Could not connection. cURL error 35: gnutls_handshake() failed: A TLS fatal alert has been received'. Get in touch with your host’s support team with the request to update their cURL library to the version 7.29.0 (minimum required version). 

2. There is no open incoming connection available on your server. To resolve the issue, request your host’s support to allow open incoming connection for your website.

3. Your WordPress Address (URL) and Site Address (URL) do not match. Make sure they are the same as your current domain name (WP Dashboard → Settings → General).