At this time the Let's Encrypt service is rate-limiting certificate issue and renewal requests for the entire *.opalstacked.com domain. This is the domain that we use to provide free subdomains to our hosting customers.
We're working with the LE team to get a higher limit. In the meantime if you are attempting to issue a LE cert for a site that uses your opalstacked.com subdomain then it may fail. If this happens you'll receive a failure notice via email.
Until the rate limit is raised then the alternative is to use one of your own registered domains on your site instead of your opalstacked.com subdomain. If you're not ready to point your domain at Opalstack, then you can create a testing subdomain and A record via your NS provider to point that subdomain at your Opalstack server.
We'll update this issue as soon as the rate limit has been raised.