Failed sending motors off command

It is hard to be sure without seeing more of the script or the messages displayed when running it.

I would suggest the most likely culprit is forgetting to call the Init function on the pbr0 object.
If you have not already it would be a good idea to call ResetEpo as well.

This would give the following initialisation code:

# Setup the library ready for use
import PicoBorgRev as pbr
import time
pbr0 = pbr.PicoBorgRev()
pbr0.Init()
pbr0.ResetEpo()