First change permission for sudoers file of the system
chmod 666 /etc/sudoers

Open sudoers file by chmod 666 /etc/sudoers

Then look for root ALL=(ALL) ALL

Add your user name to the list by inserting youruser ALL=(ALL) ALL to the file, save and close the file

Change permission of sudoers file back to ready only
chmod 440 /etc/sudoers