adding a sudoer in debian

As root edit /etc/sudoers and place the following line:

youruser    ALL=(ALL) NOPASSWD:ALL

after

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

In this way you will be capable to execute all commands that require sudo privileges passwordless.

In order to use sudo and be prompted for a password you need to remove

NOPASSWD:ALL

Following command is correct and enough but you need to completely logout the user and relogin to work .

usermod -a -G sudo user

In addition, in Debian if it doesn't work you need to install sudo package:

apt install sudo

Then you should have /etc/sudoers file and proceed as follows:

usermod -a -G sudo user