If you get one of these two messages it means you either have a bad connection or you forgot to turn off the screen that you used last (the second one only happens in my experience when use multiple types of screens).
xrandr: cannot find crtc for output VGA1
xrandr: cannot find crtc for output TV1
The easiest and quickest thing to do is to turn off the previous screen that you used like this.
xrandr --output TV1 --off
xrandr --output VGA1 --off
Then turn it back on like this.
xrandr --output VGA1 --mode 1024x768 --rate 60
xrandr --output TV1 --mode 1024x768 --rate 60
If that doesn't work you need to check connections. Running the xrandr command you should have an output like this. If you don't then it is confirmed you have a connection problem.
$ xrandr
Screen 0: minimum 320 x 200, current 2048 x 768, maximum 4096 x 4096
LVDS1 connected 1024x768+0+0 (normal left inverted right x axis y axis) 304mm x 228mm
1024x768 60.0*+
800x600 60.3 56.2
640x480 59.9
VGA1 connected 1024x768+1024+0 (normal left inverted right x axis y axis) 338mm x 270mm
1280x1024 60.0 + 75.0
1152x864 75.0
1024x768 75.1 70.1 60.0*
832x624 74.6
800x600 72.2 75.0 60.3 56.2
640x480 72.8 75.0 66.7 60.0
720x400 70.1
DVI1 disconnected (normal left inverted right x axis y axis)
TV1 unknown connection (normal left inverted right x axis y axis)
848x480 59.9 +
640x480 59.9 +
1024x768 59.9
800x600 59.9