Log in as root.
Add your user to the sudoers file.
echo 'your_plain_loginname_here ALL=(ALL) ALL’ >> /etc/sudoers
Now the error message should be gone!
One more tips for those users had Linux box behind the firewall, and you want to download software via Text Internet. You can do it in 2 ways, one is permanently add the following line in the profile, or type the following line in the terminal window: