Your boiler thinks a flame is present when it should be off, which is a safety lockout preventing the unit from starting.

False flame lockout

What causes this fault?

This fault occurs when the boiler's internal sensors detect a flame after the burner should have switched off. The most common reason is a faulty sensing electrode sending a false signal, or a gas valve that is slightly sticking and allowing a tiny amount of gas to leak through when it shouldn't.