I have a Brother 2270DW printer and was getting duplex errors like “duplex disabled”.
I found the duplex tray was totally inserted but the little actuator was bent on the right side looking from the rear of the machine. You can’t miss it once you look for it. There is a blue and red wire that go to the duplex tray switch. This switch has a function of making sure you have the duplex tray inserted fully. However, I took the side off to get a better look and discovered the switch was physically broken. It could not do what it needed to do. Since I had no spare switch and doubt I would ever find one, I pulled both wires off the switch, stripped about 1/4 inch and twisted them together. Then I put black electrical tape on the twisted bare wire connection and carefully found an area to tuck it into. I put all covers back on and now duplex works fine. Took me all afternoon to figure it out. Hope this will help someone else.