Your boiler thinks it sees a flame even when it hasn't started the burner yet, which is a safety conflict that prevents it from firing up.

Flame simulation.

What causes this fault?

This fault occurs when the boiler's sensors detect an electrical current and assume a flame is present before the gas has even been ignited. This is usually caused by a faulty ionisation electrode sending a false signal or moisture and carbon build-up creating an electrical 'leak' back to the control board. For safety, the system refuses to start the ignition sequence to prevent gas from entering a chamber it believes is already lit.