I assume you have a separate spa, meaning your spa is not connected to your pool, and has separate equipment for it.
If this is the case then your water sensor for your spa is bad, or the wire to it has been broken.
The water sensor is a little black thing that has been drilled into the return pipe at the pool equipment, and has a metal clamp holding it in place.
If your pool and spa are connected, and share equipment, then you need to change the setup of the Aqualogic to "shared equipment" which will get rid of check system light.