echo $DISPLAY

If not set, then you cannot open windows. You could try setting it with this:

export DISPLAY=:0.0 ## <<-- must match what original user sees

If it still fails after that, then there's some security or something getting in the way. To prove this, you can do this from either root or the original user:

xhost +

and then try to open the window. But this is horribly insecure for many people. Maybe Bitt can help more here now.