I've been trying for a few days to upload a song to my iPhone until finally discovering this "Manually manage" checkbox.
If it's not checked, clicking "Add to device" silently fails with no clues whatsoever.
This is the first time I'm buying an Apple product and I'm extremely frustrated with it and it's mostly because of iTunes which is an unbelievable piece of sh*t.
Not to mention I cannot even use it properly because there's no version for Linux. Maybe Apple's thinking that with tricks like that they'll make me buy a Mac. This is the last time I'm buying an Apple product.