[Koha] Unable install SUDO

Sir/Madam I have installed Debian Xfce in Virtual Box also added non-free repository in sources.list. Then updated and upgraded with apt-get. but updation is not going smoothly. then after I am unable to install Sudo and this is necessary step in koha installation. I think this is probably problem with repository. Kindly help me which repositories are required ? Rajesh Bhatt