You have two different users. One that started the desktop and one that's starting an application hat uses the desktop. When you have two different users, permissions/privileges are likely at issue. The second user needs permission to use the desktop started by the first user.
That doesn't solve the problem, but I wanted to point it out just so you have a reference. So, what you need to do is issue an xhost
command. An example I found online
It's been a while since I messed with it, but I don't think issuing that command is a permanent change. You might have to issue it each time you start a deskop. You'll need to find an appropriate X11 startup file to make the change permanent (if that's what you want).