I have an odd problem. I have tried to use Synaptic so I can install a package. The system asks for the root password but won't accept it. I can start a terminal window and start su with the root password and start Synaptic in su and it comes just fine. The same for Apper except it won't start in su for GUI reasons. 
What must I do to correct this?
I am using Debian 9 stretch with KDE.

sudo? Either way I'm guessing it's trying to usesudo. In which case it's actually prompting for your user password and not the root password. It could also be that your user is not an authorized sudoer.env 'DISPLAY=$DISPLAY' 'XAUTHORITY=$XAUTHORITY' KDE_SESSION_VERSION=5 KDE_FULL_SESSION=true apper