Depending on your server setup, accessing this resource (/nab/v1/site/free) results in a 301 redirect response. That is, your server tells your browser the requested URL is not correct and replies with an alternative URL that should be used instead: /nab/v1/site/free/. So basically your server is telling your browser to use the same URL with (and here comes the difference) a trailing forward slash.
Unfortunately, after this redirection, the browser is accessing the REST API endpoint using a GET request (instead of the original POST request). From our plugin’s REST API perspective, this is an invalid request and it simply ignores it… which means you’re stuck in this screen.
To fix this issue please contact your hosting provider and ask them not to auto-append a trailing forward slash to any WordPress REST API endpoints.