When installing debian-12.6 in Minimal Install without a graphical shell,
I gave up everything in the “software selection” section.
The lack of programs forced me to look for a “usermod” to continue my work.

Where is usermod?
Then I entered the direct path /usr/sbin/usermod

sudo is missing
su -
apt install sudo

/usr/sbin/usermod -a -G sudo username

There will be no access via ssh either

sudo apt install openssh-server
sudo systemctl start sshd

won’t work: sudo apt update

default:
deb cdrom:[Debian GNU/Linux 12.6.0 Bookworm – Official amd64 DVD Binary-1 with firmware 20240629- 10:19]/ bookworm contrib main non-free-firmware

need to execute commands

sudo nano /etc/apt/sources.list

replace text in file

This is the stable repo for the current release.
deb http://deb.debian.org/debian bullseye main
deb-src http://deb.debian.org/debian bullseye main
Next comes the security repository for installing the latest security updates.
deb http://deb.debian.org/debian-security/ bullseye-security main
deb-src http://deb.debian.org/debian-security/ bullseye-security main
And finally, stable-updates for getting stable updates until the next Point Release of the current distribution.
deb http://deb.debian.org/debian bullseye-updates main
deb-src http://deb.debian.org/debian bullseye-updates mainrity contrib main non-free non-free-firmware

sudo apt update -y

now I was able to continue installing debian.