user is not in the sudoers file. tried to solve it, same result [duplicate]

Question

I tried user is not in the sudoers file and My user is not in the sudoers file with the same result:

root@avilellaM710t:/home/avilella# sudo adduser avilella sudo         
The user `avilella' is already a member of `sudo'.
root@avilellaM710t:/home/avilella# exit a
vilella@avilellaM710t:~$ sudo ls
[sudo] password for avilella:  avilella is not in the sudoers file.
This incident will be reported. 
avilella@avilellaM710t:~$ whoami
avilella

Also, see:

root@avilellaM710t:/home/avilella# cat /etc/sudoers
#
# This file MUST be edited with the 'visudo' command as root.
#
# Please consider adding local content in /etc/sudoers.d/ instead of
# directly modifying this file.
#
# See the man page for details on how to write a sudoers file.
#
Defaults        env_reset
Defaults        mail_badpass
Defaults        secure_path="/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin"

# Host alias specification

# User alias specification

# Cmnd alias specification

# User privilege specification
root    ALL=(ALL:ALL) ALL

# Members of the admin group may gain root privileges
%admin ALL=(ALL) ALL

# Allow members of group sudo to execute any command
%sudo   ALL=(ALL:ALL) ALL

# See sudoers(5) for more information on "#include" directives:

#includedir /etc/sudoers.d

Any ideas?