[Trisquel-users] How to allow 2nd user?

[john . wilkins21]

I'm running Trisquel 6. I have added a second user with full administritve  
rights but when they try to add remove applications it asks for their  
password. They enter their password and get the message
"Failed to run /usr/sbin/synaptic '--hide-main-window' '--non-interactive'  
'-o' 'Synaptic::closeZvt=true' '--parent-window-id' '33554435'  
'--set-selections-file' '/tmp/tmpI7Bhwl' as user root.

The underlying authorisation mechanism (sudo) does not allow you to run this  
program. Contact the system administrator."

What do I do to overcome this problem please ?