I'm having trouble on this solution as well.  I'm pretty sure I've completed all of the steps but am getting the
"Could not find a properly routed case. Ensure that you've setup Omni-Channel correctly by following the requirements."
error as well.  The issue I seem to be having is that after changing ownership of the case to the queue "High Priority", I am not getting a notification in the Omni-Channel widget box to accept.  It just says "You have no active requests".  Does anyone have any suggestions?