Jetpack is not able to access the https://frontiersmp.42web.io/ site’s XML-RPC file, which is why it cannot connect. The XML-RPC file is returning a ERR_HTTP2_PROTOCOL_ERROR error.
To fix that, I recommend checking the following:
1) Can you see the xmlrpc.php file at the root of your WordPress installation when accessing your site via FTP? If you don’t, then please try re-installing WordPress. You can do this by going to the Dashboard -> Upgrades page, then click the Reinstall WordPress button.
2) Do you use any security plugins that may block access to this file? If so, could you try to disable them? You might also want to check your site’s .htaccess file for any rules that might be blocking access to the?xmlrpc.php file.
3) Does your hosting provider block access to this file? If you don’t find any plugin that may block access to the file on your site, I would recommend that you get in touch with your host. If they only offer XML-RPC access by allowlisting IP addresses, then they will need to allowlist the Jetpack IP addresses listed here:
https://jetpack.com/support/how-to-add-jetpack-ips-allowlist/
Once Jetpack is able to access your site’s XML-RPC file, you should no longer see any connection errors and the plugin should work normally.
