DIAGNOSIS You have a sealing failure. The silicone sealing ring is likely mis-seated, worn, or the lid isn't fully locked.
FIX Turn the unit off. Open the lid. Remove the sealing ring. Clean the ring groove and the ring itself. Reinstall the ring, ensuring it's fully seated in its groove all the way around. Close the lid. Align the arrow on the lid with the unlock symbol on the pot. Turn the lid clockwise until the arrow aligns with the lock symbol. The float valve should be up before you start cooking.
IF THAT DIDN'T WORK Check the float valve itself. Remove it by pushing up from the bottom. Inspect for food debris clogging the hole or the valve stem. Clean it. Ensure the little silicone cap on the valve is present and intact.
HEADS UP Never force the lid closed. If it won't lock smoothly, the ring is out of place. Forcing it can damage the locking pins.