How to add a user to sudo?

Question

linux

I am using Debian.
I write su, I enter the password.
I write sudo usermod -a -G sudo user(without sudo writes bash: usermod: команда не найдена).
In general, nothing happens after the command and the user is not added.
What to do?

Problem solved, solution in comments

Answer 1

INSTALLING SUDO IN DEBIAN 10