Your boiler has detected that the water pressure inside the system is currently too high.

DIY fix possible

Locate the external filling loop, usually a small metal braided hose under the boiler.Ensure both valves on the filling loop are fully closed to prevent extra water from entering.Drain a small amount of water from a nearby radiator bleed valve until the pressure gauge on the front of the boiler returns to the green zone, typically between 1.0 and 1.5 bar.