Try pressing down on the slide lock mechanism and see if that lets you move the take down lever.
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Yep this was the problem. The takedown safety level hooks around a little pin in the rear end of the FCU. This little pin is what move the sear up/down. Somehow the takedown level must have lost its grip to the little pin. The fix is to move the takedown level out a little bit so the little pin can go forward when pressing down the sear. Then move the takedown level back in position. Make sure that the takedown catch the little pin.Your takedown safety lever is out of position. It should be holding the sear down.