try using a non root user, like makeuser notroot sudo notroot do that again OR if you know what you are doing use -r
While you can do that, it's highly discouraged. You should never run any thing as root, as that allows programs to change (or even break) the system. Instead, you should run PocketMine as a user without root access like @Thunder33345 suggested. The root user should only be used to make system modifications.
Also, a more descriptive title would be nice. I saw this and thought it was something completely different (One of the other billion 'Help' threads). Putting maybe a snippet of the error (In this Case Maybe: "Root User Error" or "Program Running as Root") would greatly help others to give you aid for your problem. Since your thread is posted in the "Help" section of the forums, we assume that is what you need Like what was said before, this is very dangerous. You never want to run something random from the internet with -r. That gives it access to ALL of your files, and potentially break your system. (I made that mistake once, bad idea. Bricked my system. Good thing it was a Raspberry Pi :b )