Your boiler has detected that the water pressure in your heating system is currently set too high for safe operation.

DIY fix possible

Locate a radiator in your home, preferably one upstairs.Place a cloth under the radiator bleed valve and use a radiator key to open it slightly.Release a small amount of water until the boiler pressure gauge drops to the green zone (typically 1.0 to 1.5 bar).Tighten the bleed valve firmly and check if the error clears.