Superadmin account cannot be removed directly.
You need to change its role to Admin or any other prior to removing.
To do that, open the PostgreSQL connection to the Headwind MDM database, then run the command:
update users set userroleid=2 where login='....';
(use the login of the superadmin account in the expression).
This will make this user an Admin instead of Superadmin, and it can be removed by another admin.