welcome to ja.
Did this code show up after you washed the filter? or before?
FL codes are related to low flow. Generally becuase of clogged filters, or it could be low water levels. When air enters the pump it creates surges in the lines and will kick the pressure sensor.
When the pressure sensor or flow sensor kicks, it will not allow your heater to turn on, this is to protect the heater from dry firing and burning out.
My recommendation is to remove the filter and see if the spa will run with it out. If so you will have to replace the filter with a clean dry one that allows more flow.
If you continue to get the FL code, then you will need to look at replacing a sensor, if your pumps seem to be working correctly.