DIAGNOSIS That's the machine telling you it's unhappy with its water management. The most likely cause is a clogged or incorrectly seated dirty water tank sensor.
FIX First, power it off. Remove the dirty water tank and give it a thorough rinse—pay special attention to the little sensor nub inside the tank and the corresponding port on the main unit. Make sure the red float inside the tank moves freely and isn't stuck. Dry everything completely, reseat the tank with a firm click, and power it back on.
IF THAT DIDN'T WORK The beeping red light can also trigger if the clean water tank is empty or not seated perfectly. Pop it off, check for any debris on the seal, fill it with fresh water, and click it back in until it's flush.
HEADS UP Never run this thing with empty tanks, even for a quick dry vacuum. The pump can suck air and the sensors will throw a fit. It's a wet vac first, even when you're not using water.