I have an older Sundance hottub/spa and when I get that error it is after refilling. I believe it means an air bubble in the pipes, which can happen if you don't fill up from the filter area directly into the pipes first IIRC?
I used to put the host directly into the input port and let the water in the hose push thru (and hopefully it pushes the air thru the system too). Try that.
If that doesn't help check the manual out. You can find it online via Google.
Just found this in my manual-
"FLOW SWITCH (Heater is deactivated. The filter/
circulation may also be deactivated). Proper
flow of water is inhibited or a flow switch has malfunctioned.
Check for proper water level and for
clogged filter (page 51-52). Contact your dealer or