Your boiler thinks it still sees a flame even though it has commanded the burner to turn off, which is a safety concern that prevents it from restarting.

What causes this fault?

This fault occurs when the boiler’s safety sensors continue to detect a flame for more than five seconds after the gas supply should have been cut off. The most common reason is that the gas valve hasn't closed fully or the electrode that monitors the flame has become damaged and is sending a false signal. Because this is a major safety concern, the boiler will automatically shut itself down to prevent any risk.