(user) is not in the sudoers file. This incident will be reported.

Ubuntu 16.04 LTS

sudo usermod -aG sudo,adm (username)

exit terminal and re-open

OR

Step 2. Entered root using command su root

Step 3. Input root password

Step 4. Input command apt-get install sudo -y to install sudo

Step 5. Add user to sudoers file by inputting adduser username, put your username in place of username

Step 6. Set the correct permissions for sudoers file by inputting chmod  0440  /etc/sudoers

exit terminal and re-open