journalctl command reports OS and system service logs by extracting them from the systemd journaling system.
systemd is the fairly recent approach to managing OS and system services in modern Linux distros, and as part of streamlining services it also improved the way reporting is done – standard and error logs are now managed by systemd journaling system that’s a centralised system queried by journalctl command.
IMPORTANT: journalctl requires root access
Just like standard users couldn’t access OS and system service logs prior to systemd implementation, they still can’t with systemd.
You therefore must use journalctl with sudo:
Reverse journalctl logs
As you noticed from previous example, by default journalctl starts showing oldest logs it has. This may not be very useful.
Here’s how to make journalctl show the latest (most recent) logs instead: use -r option (-r for reverse).