To add the user to the sudoers group (as root)
usermod -aG wheel username
If it doesn’t work yet, edit the /etc/sudoers file (as root) using visudo
visudo
Uncomment this line (there should be two similar lines for wheel, make sure to uncomment this one and not the other one). Remove the # if it is there (do not remove the %)
%wheel ALL=(ALL) ALL
You can sudo now!
If you are looking to add your user to the Sudoers Group in Ubuntu/Debian, here is how to do it.