Auto-shutdown of endless OS

Hello, I need way to auto-shutdown endless OS. Does exist any app or do I need to use cron ? Also I wanted to use crontab, so I tried to install it, however every time I did it, this message appeared: "Not using locking for read only file /var/lib/dpkg/lock”. I mean shut it down at specific time [17:00] every day.

You won’t be able to install things like cron, but you can create a systemd timer unit to do the equivalent.