Insufficient Context for Simulation

What causes the message "Insufficient Context for Simulation.  Please add an item or location filter or level to the worksheet before running simulation".    This message occurred while running a worksheet that only contained an item level.  However it does not appear consistently.   After receiving this message,  a location filter was added to the worksheet and the simulation completed successfully.  I then removed the location filter and ran the worksheet again with only an item dimension and simulation then worked successfully.  I was then unable to recreate the message.