vncserver-virtual did not write out a display number

Hi,

I deployed the vncserver-virtuald daemon on centos7,but I can't connect successfully via vnc viewer, im getting this error:

vncserver-virtuald: DaemonConnection: server launch failed: vncserver-virtual did not write out a display number

vncserver-virtuald: Connections: disconnected: xxx::57938 (UDP) ([ServerLaunchFailed] Could not launch VNC Server)

What does the error message mean, and what should I do to fix the problem ?

Regards,

Jim