Debian 12: User calls synaptic: Unable to authenticate root

[Roger Price]

I installed Debian 12 and defined a user.  Now I would like to install other 
packages as that user.  As with Debian 11, a synaptic popup asks for the super 
user password, but after entering root password I get "Authentification 
failure".  This worked on Debian 11.

I click on "details" but all I see is "Action: com.ubuntu.pkexec.synaptic".
Is there some way on Debian 12 of authorising regular users to use synaptic 
(provided they know the root password)?

Any hint or suggestion would be very welcome.  Roger