Your boiler thinks it can see a flame even though it hasn't started the ignition process yet, which prevents it from running for safety reasons.

Flame signal without flame present – air/gas ratio control valve may need replacing as might the ionisation cable

What causes this fault?

This fault occurs when the boiler's internal sensor detects an electrical signal signifying a flame at a time when the burner should be off. The most common reason is a faulty detection electrode sending a false signal or a gas valve that is leaking slightly, allowing a small amount of fuel to stay lit. Because the boiler cannot safely confirm the fire is out, it shuts down to prevent any risk of overheating or gas build-up.