With the jets/pump running, check and see if the board is sending 220v to the circ pump. If it is, then shut the pumps off and see if 220v is then still going to the circ pump with the spa calling for heat.
Another trick, take the leads from the circ pump and hook the wires direct to each of the 110v legs coming into the spa. You don't want to leave them there forever, but that by-passes the board. If the pump runs, the flow code should go away and the spa should start heating. Then it's possible there is something else going on with the board or? If it doesn't run, then I would start with replacing the circ pump and go from there.
You should also confirm the topside is not in some kind of hold or goofy economy mode setting.