Not yet released for the Pixelbook.
It's been several days and V76 has still not been released for the Pixelbook. I wonder why it takes so long to be released on Google branded Chromebooks?
The update became available for the Pixelbook the evening of 8/16 and I installed on 8/17. Enabled the virtual desktops flag. Observe that the touchpad gestures to switch between are not yet working, need to do a three finger swip up or press the multitask display button to display the desktop options, then click the one I want to switch to.