Please use the search function and read the let's encrypt FAQ page here in the forum. We explain this about once a day, so I will not explain it again now.
The problem was caused by my attempting to enable SSL for a site before the domain was pointed to the server. I couldn't fix the SSL problem so I deleted the site and set it up again after the domain was resolving correctly--that worked. You might put in the FAQ/instructions: Do not enable SSL for a site unless the domain is already pointing to the IP address of the server OR in ISPconfig you go to System/Server Config/Web/SSL settings and check "Skip Lets Encrypt Check"
