DIAGNOSIS: The lid's locking pin isn't aligning with the slot in the pot body. This is almost always because the inner pot isn't seated perfectly or there's debris in the track.
FIX: First, take the inner pot out. Look at the black plastic hinge on the underside of the lid—make sure it's clear of food bits. Now, re-seat the inner pot. Don't just drop it in; make sure it's rotated so the handles align exactly with the notches in the base. You should feel a slight drop and it won't spin. Then place the lid on and turn clockwise until you hear a distinct click, not just beeps.
IF THAT DIDN'T WORK: Check the silicone sealing ring around the lid. If it's popped out of its groove or is twisted, it can block the lid from seating deep enough to lock. Remove it, clean it, and stretch it back into the channel evenly.
HEADS UP: Never force it. The beeping is the sensor telling you it's not aligned. Forcing it can break the plastic locking mechanism. If the pot is even slightly off-center, it won't work.