The response is very clear. Anoyhet user is tunning a process which blocks unelevated shutdown. You can override it with the command privided in the response. Like mentioned in a peer comment, sudo shutdown now would resuly in the immediate shutdown of the system, ignoring any interrupts. If anything else fails, interrupt power. It should start fine.

It could be a process you are running under a different user (safe), which demands you to shut it down cleanly.

Linux provides the mechanics to prevent the problem you’re angry about.