DIAGNOSIS The bucket isn't making proper contact with the float switch inside the unit. This is almost always because it's not seated perfectly, or there's a bit of debris or a warped bucket causing a misalignment.
FIX 1) Take the bucket out and set it aside. 2) Look inside the empty bucket compartment. You'll see a small, usually orange, plastic float pin in the center-back. Gently press it up and down a few times with your finger to make sure it moves freely and isn't stuck. 3) Inspect the bucket itself, especially the back corner where it meets that pin, for any plastic molding flaws or cracks. 4) Now, re-insert the bucket slowly and firmly. Push it straight in until you hear and feel a definitive *click*. Apply pressure to the front of the bucket, not the handle. The beeping should stop immediately.
IF THAT DIDN'T WORK If it still beeps, the float switch mechanism inside the unit might be defective. Try a simple test: with the unit unplugged, hold that orange float pin in the UP (bucket full) position with one finger, then plug the unit in. If it still beeps and shows the red light with the pin held up, the switch is faulty.
HEADS UP Don't just yank the bucket out by the handle. Push the release lever and lift it straight out. Slamming it in can misalign the float pin over time.