Certain security settings on your website could prevent us from validating your license. For example, if you are using Cloudflare, you will likely need to disable the blocking of external HTTP requests from your Cloudflare account.
Further, If you are using other security plugins, try disabling them one at a time while at the same time trying to activate your license.
If you are still having problems after doing all this, please contact support. Be aware that we will likely need Administrator-level access to your site in order to properly diagnose and resolve the issue. Here are instructions on how to provide Administrator-level access.