When installing a free Let's Encrypt certificate the following notification may be displayed:
In the first case, it is enough just to wait until the cache in the system is updated. DNS, and try to install the certificate again later. As a rule, it takes several hours to update the cache (in rare cases, up to 24 hours). With more details about the cache DNS can be found here.
In the second case, you need to specify in the address records of the domain the current IP addresses of the hosting account... After making the changes, it will also take some time to update the cache in the system DNS.
If the domain is served on third-party NS, then the changes need to be made not in our panel, but in the panel of the owner of these NS. To direct a domain to our hosting, you can use instruction.