Did you run discourse-setup? It will check if your DNS points to your server.
If your DNS is correct and points to your server then your issue is likely that you tried a bunch of times when DNS was not working and you have hit let’s encrypt rate limits. The easy solutions are to wait a week or to use a different subdomain. A harder solution is to request a new cert with multiple domains with Set up Let’s Encrypt with multiple domains, but since you’re already having trouble, I’d recommend you just use a different subdomain until you get things figured out.