Troubleshooting

Named after the error you were given: what it means, and what to do about it.

Could not get lock /var/lib/dpkg/lock-frontend

apt refuses to run because something else is already installing. Modern apt tells you exactly which process, and deleting the lock file is the wrong fix.
The following packages have been kept back

apt-get upgrade refuses to install new packages, so an upgrade that needs one is held back instead. What the message means and the three things that cause it.
The repository is not signed

apt cannot verify a repository's signature: the NO_PUBKEY and Missing key errors. What Debian 13 changed, and why apt-key adv is no longer the fix.
sudo: command not found

On Debian, sudo is often simply not installed, and your user is not in the sudo group. Why the installer does that, and how to fix it from a root shell.