DIAGNOSIS The blender's computer has detected a fault, usually related to the motor, a sensor, or an overload condition. It's locking you out to prevent damage.
FIX First, unplug the unit from the wall for a full 60 seconds to reset the system. Plug it back in. If 'err' is still there, it's likely a thermal lockout. Let the unit sit, unplugged, in a cool spot for a few hours to let the internal components fully cool down. After that, try a simple low-speed pulse with just a cup of water in the jar to see if it clears.
IF THAT DIDN'T WORK If the error persists after a full cool-down and reset, the issue is often with the jar not being correctly seated on the base. Take the jar off, clean the metal contact ring on the base and the matching contacts on the jar with a dry cloth. Make sure the jar is twisted all the way until it clicks firmly into place.
HEADS UP Don't ignore this error and try to force it to run. The 'err' code is there for a reason, often to protect the motor from overheating due to a jam or a faulty temperature sensor. Running it in an error state can void your warranty.