first keep in mind that a user is who has access to odoo using email and password. so Administrator is a user that has highest access to odoo, by default Administrator is also a user when we run odoo at the first place  and open the Employees there is administrator right there, when we create new employee and assign it to POS (multiple user per pos) than we know that he can't close the session, and if we try to add Administrator , also Administrator can't close the session . but why ? it's because Administrator here act as employee , to grant access as user you need to linked him as administrator in HR Settings -> Related User -> Administrator

Yes but in Version 17 it is not showing the close option even I am the admin. Only two options are visible Order and Lock. I cannot close the POS session although I am admin.