Comments are blocked by default on WordPress installations when the Sucuri Firewall is activated. This is done by blocking access to the xmlrpc.php file.
This can cause issues with allowing users to comment on your posts, and also with other plugins or applications that make use of the WordPress xmlrpc.php endpoint. If you'd like to allow access to your xmlrpc endpoint, you can easily change this default setting.
1) Log in to your Sucuri Firewall dashboard here. click on the settings gear icon and then on Security
2) Locate the "XMLRPC, Comments and Trackbacks blocked." option and uncheck it.
The disabled setting looks like this: