Understand the JK Flip Flop Logic Diagram
Your simulator is running in to a what was first, the egg or the chicken? type scenario because to determine the output of the system the internal state of the gates must be resolved but because those internal state use the outputs the simulator cannot determined any of those states. As you can see the JK flip-flops you showed in the schematic they omit the circuitry for the CLR pin which would force the JK flip-flop into a known state