My customers enabled Shared SSL for his domain and created 3rd Level Domain Alias, however when he goes to the page by https it fails to be displayed saying it “can’t find server”.

Make sure the Domain alias A DNS Record resource isn’t disabled for this plan, as it is essential for 3rd Level Domian Alias which in its turn is required to enable Shared SSL. DNS zone for this 3rd Level Domain Alias should contain two built-in records pointing to the IP address of the main domain.