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

Flame detected with the burner off – electrode defective main board defective

What causes this fault?

This fault occurs when the boiler's internal sensor detects an electrical signal mimicking a flame before the heating cycle has actually started. This is usually caused by a carbon-coated or damaged electrode sending a 'false' signal, or moisture and debris on the burner assembly interfering with the detection system.