After reviewing your site’s source code, we don’t see any reason that the SEO test should return that message. We ran other random sites through the test and do not see that result regardless of whether or not they use our plugin.
This means the plugin outputs correctly and the SEO test sees the tags on other sites which means something within your specific setup is likely causing the issue.
Sometimes strange behavior may be caused by a plugin or theme conflict. The fastest way to rule out conflicts is to deactivate all non-Yoast plugins and switch to a standard theme like Twenty Seventeen. If this resolves the issue, you have a conflict and should activate one plugin at a time until you locate the conflict. Finally, after all plugins are active, switch to your theme. This will narrow down the conflict to a specific plugin or theme.
If you are unfamiliar with checking for conflicts, we wrote a small guide that will walk you through the steps. You can find it here: How To Check For Plugin Conflicts