To solve the problem when enabling Let’s Encrypt SSL at VestaCP for a domain error prompt Field “ssl certicate, ssl key” can not be blank.

Goto server console and edit this file.

find “LETSENCRYPT” same line with the domain name that facing problem
set LETSENCRYPT to “no” and save the file.

Back to VestaCP, edit the domain, enable the SSL and save the domain.