Error when attempting to create Python GUI using Tkinter: “no display name and no $DISPLAY environment variable”

Sup,

• I am simply trying to use Tkinter (a Python GUI creator) to create a GUI on my Raspberry Pi.
• To start, I only want a GUI to show up on my screen. That's it!
• The code DOES work on my PC
• The code does NOT work on my Raspberry Pi

The entire script looks as follows:
(works on my PC, bringing up an empty GUI)

import Tkinter

root = Tkinter.Tk()

root.mainloop()

Trying to run the program in terminal as follows:

sudo python GUI.py

Produces the following error:

Traceback (most recent call last):

File "GUI.py", line 4, in root = Tkinter.Tk()

File "/usr/lib/python2.7/lib-tk/Tkinter.py", line 1817, in init

self.tk = _tkinter.create(screenName, baseName, className, interactive, wantobjects, useTk, sync, use)

_tkinter.TclError: no display name and no $DISPLAY environment variable

Does anybody have any idea how I can resolve this issue? The error is produced under all of the following conditions:

• Direct HDMI to computer monitor
• DSI connection to touchscreen LCD
• SSH on computer monitor

For reference, if I type the following into my terminal:

echo $DISPLAY

An empty string gets printed, so I believe therein lies the problem

The following commands don't work either, which might shed light on the problem:

sudo apt-get update

startx