I've recently installed Debian 9 and encountered the following error.
During installation I've set up 'root' password and I've set up 'user' with his own password. Later when I log into 'user' account and want to install some package I have this problem. If I run:
sudo apt-get install 'package'
then I get this message:
'user' is not in sudoers list
And if I try to log into 'root' terminal with:
and enter password, I get:
su: Authentification error
P.S. I understand that question may be really silly, but I've not found any information about it in internet, so I need to ask it here.