Please follow the following steps:
1) Login to Plesk control panel.
2) Click on Subscriptions.
3) Select the corresponding Subscriptions from the list.
4) Click on Websites & Domains.
5) Click on DOWN Icon as mared in RED.
Show/Hide the list of domain-specific operations (like installing SSL certificates and apps, adjusting DNS settings, and other)
6) Click Remove Subdomain.
7) Click Yes.
Information: The subdomain was removed.
