The flow switch has failed assuming that the pump and filter are working properly. This is the safeguard feature to prevent the cell from turning on without water flowing over it.

Have you cleaned the salt cell recently?

Under the cell is a disk with a thin cable running to it. Turn off the pump and and salt systems power and remove the union or flange that holds the disk. On the bottom of the disk you will see 3 silver contacts take a scotch brite pad and clean them ... Reassemble and see it the flow lite goes off if not you will need to replace the disk ( flow sensor) .

Under the cell is a disk with a thin cable running to it. Turn off the pump and and salt systems power and remove the union or flange that holds the disk. On the bottom of the disk you will see 3 silver contacts take a scotch brite pad and clean them ... Reassemble and see it the flow lite goes off if not you will need to replace the disk ( flow sensor) .