Ok, I see. In our current appliance version "parallels" user is not added into admins in policykit :-( So I suggest you to set up root password and use it for authentication as Administrator. For this do in terminal:
sudo passwd
First it will require you to enter your current password of "parallels" user:
[sudo] password for parallels:
and then new password for root:
Enter new UNIX password:
Retype new UNIX password:
- specify good complex password. After that when Debian's GUI asks you to authenticate as Administrator - enter root's password.