visudo command not found! error message on Debian when you try to add a user to Sudoers

if you have the following error:

erbcuser@debian:~$ su
Password: 
root@debian:/home/erbcuser# visudo
bash: visudo: command not found
root@debian:/home/erbcuser# 

try to execute the following command:

dpkg -S visudo

then:

su -c 'apt-get install sudo'
su -c 'gpasswd -a erbcuser sudo'