Cfxtrjtrk

im adding one user to the sudoers file and when i enter the command sudo adduser <username> sudo, shows error <username> is not on the sudoers file. this incident will be reported.

Any suggestion what could it be? why that error if im trying to add.

You are trying to use sudo , however you do not have the right to do so as you yourself are not a member of the sudo group. In such circumstances you will always get that error message when you attempt to use sudo for whatever reason. Example (notice that richard is not a member of sudo):

richard@s15:~$ groups

richard adm cdrom dip plugdev lxd lpadmin sambashare libvirtd

richard@s15:~$ sudo ls -l

[sudo] password for richard:

richard is not in the sudoers file.  This incident will be reported.

Now, that same thing for me (notice that I am a member of the sudo group):

doug@s15:~$ groups

doug adm cdrom sudo dip plugdev lxd lpadmin sambashare libvirtd

doug@s15:~$ sudo ls -l

total 4112

drwxrwxr-x  2 doug doug   4096 Oct 25 16:37 audio

drwxr-xr-x 39 doug doug   4096 Oct 14  2016 backup_carrie_01

...

Furthermore, since I have the rights to do so, I can add richard to the sudo group:

doug@s15:~$ sudo adduser richard sudo

Adding user `richard' to group `sudo' ...

Adding user richard to group sudo

Done.

And for a subsequent login for richard:

richard@s15:~$ groups

richard adm cdrom sudo dip plugdev lxd lpadmin sambashare libvirtd

richard@s15:~$

Try to add the user to the sudoers file manually:

• Go to home directory by running command cd
  


• Then type sudo su
  

It will ask you to type your password to be in root user.

• Run visudo
  


• 
  

  Then add this line to the end of the file (change username with your username:

  
  
  

  username        ALL=(ALL)      ALL
  
  

  
  



• Save and exit

Credits:
Username is not in the sudoers file.