DIAGNOSIS The constant beep is a failsafe alarm because it thinks the water reservoir isn't seated properly or is empty. It's the machine's way of saying "I can't start without water."
FIX First, make sure the removable reservoir is pushed all the way in until it clicks. It can look seated but be off by a millimeter. Pull it out, check that the rubber seal is clean and flat, then push it back in firmly. If it's full of water, also check that the float inside the reservoir is moving freely and not stuck.
IF THAT DIDN'T WORK The water level sensor inside the main unit might be blocked by mineral scale. Unplug the machine, pour out all water, and use a 50/50 mix of white vinegar and warm water. Let it sit in the reservoir and the main water basin for 20 minutes, then run a few cycles with the vinegar mix to flush the sensor area.
HEADS UP If you're using bottled or filtered water, the float can sometimes get sticky from additives. A quick rinse under the tap fixes it. Don't just ignore the beep and try to force it to run—it'll damage the pump.