Cannot enable Shared SSL in Plesk 11.5 and higher.
Admin has no its own subscriptions
Master SSL domain in not configured properly
Check first that Plesk
adminhas its own subscirptions.
NOTE: This option is designed for admin only and only admin subscription domains can be used as Shared SSL after 11.5 version of Parallels Plesk
Check that Shared SSL configured properly:
2.1. Enable SSL on domain in Website Scripting and Security that should be shared.
2.2. In Websites & Domains of the same website go to Master SSL Domain and click on Switch on shared SSL
2.3. Go to Tools & Settings > Shared SSL and select the Switch on shared SSL checkbox.
2.4. Select the required website from the Domain name menu.