Hi Ms Jenna, welcome!
The Pool Pilot will display the Check System light for any warning conditions. For the Purifier Off, Check Flow, it means there's not enough water flow going to the manifold assembly. This can be due to several reasons. The easiest ones to check are the following:
Skimmer and Pump Baskets.
Diverter valves to the pump.
Make sure the pump is on and moving water through it by inspecting the pump basket area, which should have a clear lid to see into the trap.
Check your filter pressure to make sure you don't need to backwash or clean your filter.
Check all valves between your pump and AutoPilot manifold to make sure there aren't any that are closed or diverting flow from the manifold.
At the manifold, make sure the tri-sensor cord is plugged into the tri-sensor.
Turn off the pump and loosen the manifold union, where it has a label that says "Clean Union Screen Regularly". There is a mesh screen to catch debris, and can get clogged.
OK, that's a start.