SSL subdomain problem

Good morning.
There seems to be a problem with one of my subdomains.
When I search my subdomain (e.g.: xxx.domain.gr) under https, the site runs securely. When I search the second-level subdomain (e.g.: xxx.xxx.domain.gr) under https, a warning message appears “This site can’t provide a secure connection”.
What could be the problem?
Thank you in advance.

Good morning and thank you for your reply.

I would like to ask if the 10$/month subscription covers also a fourth level. If, for example, the address is zzz.xxx.domain.gr, I will be covering the xxx, but will all the zzz subdomains be working securely as well?

Thank you in advance.

According to the article I linked to, yes.