I have investigated your issue further and I see the following error when validating your site.
cURL error : Could not resolve host: theme.co
Please make sure that your PHP cURL extension is enabled on your server and functioning properly. You can check the following resource for more information (https://kb.yoast.com/kb/is-my-curl-up-to-date/).
Then the next possible issue is that your firewall mayt blocking the requests to theme.co. You will have to contact your hosting provider to check that issue.
We also have a troubleshooting guide on how to resolve connection issues when validating your site (https://theme.co/apex/forum/t/troubleshooting-validation-connection-issues/201).
Let us know how it goes.