You are logged into a Linux console in some unique fashion, e.g. "The Proxmox web console of an LXC guest".
You want to start a service, as a REGULAR USER, using the forever 🤬 CURSED systemd in the correct fashion (i.e. "in the user's process namespace").
Since you're logged in with root on the Proxmox web console of the guest LXC and still setting things up, you su - $user in order to get a login shell session of the target user:
root@vinecraft:~# su - minecraft