[ubuntu] Screen resolution problem: xrandr failed to get size of gamma for output default

Hi to all,

I installed ubuntu 15.10 as guest O.S. on virutalbox.

I have a problem with the screen resolution because I want set a resolution of 1024x768 but the max resolution that ubuntu retu me is 800x600.

I tried to increase the max resolution to 1024x768 using the "XRANDR" command, but this command not work properly.

If I run the command I get the following error (xrandr: Failed to get size of gamma for output default):

user@Ubuntu:~$ xrandr --q
xrandr: Failed to get size of gamma for output default
Screen 0: minimum 640 x 480, current 800 x 600, maximum 800 x 600
default coected primary 800x600+0+0 0mm x 0mm
   800x600       61.00  
   640x480       60.00

Please, can any one help me to resolve this problem?

Thanks in advance!