DIAGNOSIS The lid lock microswitch is not being fully depressed. This is a safety interlock failure. Debris or a misaligned lid is the most common cause.
FIX Unplug the unit. Remove the lid. Look at the small black plastic nub on the front underside of the lid. Now look inside the base at the corresponding hole. Use a dry cotton swab to clean out any dried mix or debris from that hole. Wipe the lid nub clean. Re-seat the lid firmly, pressing down on the front until you hear a distinct click.
IF THAT DIDN'T WORK The microswitch inside the base may be physically broken. You can test it. Unplug the unit. Use a multimeter to check for continuity on the switch when you manually press the actuator inside the hole with a pen. If no continuity, the switch assembly needs replacement.
HEADS UP Never force the lid or try to bypass the switch. The blade will not spin without this circuit being closed. A persistent error means internal repair is required.