The heating failed message means that the Ultimaker can't heat up the nozzle. When you get this error message you will probably have a flashing UltiController display as well.
The reason for this heating problem is most likely a bad connection. Therefore you need to take a look at the heater cartridge cables; they are connected to 'HEATER1' on the main board. It is advised to take them out and reconnect them to ensure they're properly connected.
If the error still appears after having checked the connections, it could mean that the heater cartridge itself is damaged. Contact one of our local support teams for further assistance.