It was interesting, made me try using sudo, and learning a little more about it,
we do get a lot of questions on this. Also, if the OP does check in, please edit the subject line in your first post, add "solved" to it, and it might be better to change the subject, from "Terminal issue" to something like "sudo not working",
Any way, another thing that was kind of odd, unexpected, when I started my
Debian VM, to try some of the commands, I thought I would need to install sudo, since I had never installed it, but when I tried 'man sudo', the manual was there, and 'man sudoers', etc. When I tried the 'usermod' command, it appeared to work, and the 'grep' command showed I had been added to the sudoers group.
But then when I tried actually running a command as "sudo", that is when I got the "command not found" message, installing sudo solved that, but then I still had to repeat all the other steps, to add myself to the group.
That is what made me think the OP also, had not installed sudo, it turned out they had, but any way
To sum it all up, the first thing any one should do, is make sure sudo is installed,
Should work , if it is not installed, install it.
then follow the rest of the commands to put the user in the sudoers group
More details here:
https://wiki.debian.org/ListInstalledPackages
and
https://www.cyberciti.biz/faq/find-out- ... -in-linux/
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https://wiki.debian.org/sudo