DIAGNOSIS The Instant Pot isn't detecting that the lid is properly closed and locked. This is almost always a contact issue, not a broken part.
FIX 1) Unplug the base for 30 seconds to reset it. 2) Place the empty inner pot inside. 3) Look at the small metal pin on the inside of the lid's handle—that's the lid sensor. Now look at the corresponding hole in the pot's rim. Wipe both with a dry cloth. 4) Align the arrow on the lid with the 'unlocked' padlock symbol on the base. Close the lid. 5) Turn the lid to the right until the arrow points to the 'locked' padlock symbol. You should hear a distinct click. The 'Lid' message should disappear.
IF THAT DIDN'T WORK The silicone sealing ring might be out of its track, blocking the lid from seating fully. Remove the ring, stretch it a bit, and reseat it perfectly in its groove around the underside of the lid. Then try locking it again.
HEADS UP If the lid ever feels too loose when it's 'locked,' the plastic locking pins on the underside of the lid might be worn. That's a call to customer support, but it's rare.