Android File Transfer cannot connect to device from Mac
My MAC is running MacOS 10.12.2, and my phone is an S7 Edge running the UK release Nougat 7.0. With Android File Transfer (AFT) I was also getting the error "Could not connect to device. Try reconnecting or restarting your device.", until I uninstalled Kies on the MAC. Thereafter AFT connected to the device and I was able to transfer files from the phone simply by dragging them to a(ny) folder on the MAC.

I have USB debugging enabled, MTP mode selected in phone's settings, and I had to respond "Allow" to a pop-up on the phone.