Have you tried doing a PID tuning on it? This should be done when adding any heater or changing anything with a heater.
Also, make sure you set it to the correct thermistor.
What this error means is that it thinks it has lost control of the heater. For instance, if the heater falls in temp and takes too long to recover, this will trigger. If it takes too long to heat up, this will trigger. There are settings to configure the time space allowed for the recovery and heat up but I don't mess with Marlin very often and don't remember what they are.
I hope some of this info is useful.