When attempting to apply a Let's Encrypt certificate to a subdomain, Plesk shows one or two warnings:
This occurs when the parent domain has a wildcard SSL certificate installed. For example: You want to install a certificate on subdomain.mydomain.com and you have, in the past, generated a wildcard certificate for mydomain.com
The solution is to simply apply the wildcard certificate to subdomain.mydomain.com. Here's how:
After about 90 seconds the certificate should be applied successfully.