User is not in the sudoers file. This incident will be reported.(Ubuntu server 18.04)

I found this problem on ubuntu server 18.04, okay now we go to problem

Go to root user

su

Step 2 go to nano /etc/sudoers

nano /etc/sudoers

and add the following line (after the root line ALL = (ALL) ALL):

username ALL=(ALL) ALL

last, save ctrl+x and press y , enter

Good luck