VNC black screen

[milen-prg]

I installed proot Arch Linux with XFCE4 by instructions here. The easy script from here works and I can use the desktop in x11 server application. But it is not convenient, so I want VNC.
I add xfce4-session & in xstartup, but the screen in VNC client is black.
The log is:

xfce4-session: Cannot open display: .
Type 'xfce4-session --help' for usage.

Using startxfce4 have the same result.