The lavs have a pump that generates a vacuum on the ground and below 18,000 feet. If that pump is inoperative which happens more than it should the lavs will not have enough vacuum to flush. Above 18,000 differential pressure from the cabin pressurization system flushes the lavs. You can dispatch with the pump inop but the lavs are required to be locked out below 18,000 feet.
note: The actual altitude varies between aircraft types but is usually 16,000 feet. 18,000 feet is used because it’s the transition point for switching altimeter settings which pilots use as a cue to call the flight attendants and lock the lavs.