Make sure the pressure release valve is turned to the SEAL position. If you’ve done this and still hear a loud hissing noise, it may indicate your Silicone seal is not fully in place.
Press START/STOP to stop cooking, VENT as necessary, and remove the pressure lid. Press down on the Silicone ring, ensuring it is fully inserted and lies flat under the ring rack. Once fully installed, you should be able to tug lightly on the ring to rotate it.