This is a simple fix if you get the following error.
It's cause when Let's encrypt cant contact the domains linked, so if you set everything up and it's not working here is the fix. (if you are using Cloudflare also check that you have the ACME challenge rule applied, see https://tree.nodehost.ca/d/131-make-let-s-encrypt-work-when-using-cloudflare-acme-challenge/9)
Visit your domains area on the panel
If you use a sub domain for the site address it will auto add www. to the domain, you can delete it or add the www. to your DNS if using a external DNS. (If internal this is NOT the reason for the Let's Encrypt failure as we do add the www. alias to the internal DNS also.)
Just enable Let's Encrypt again
Once this is done you should be able to enable Let's Encrypt. If you still cant enable create a discussion on the community so we can take a look.