Can’t transfer songs to iPhone
If you have experienced issues where you were unable to copy or transfer your songs/mp3 files to your iPhone device using drag and drop feature, here’s what you need to do.
Open iTunes and connect your iPhone. In the left pane click on the iPhone and make sure that Summary tab is selected.
Scroll down and check the “Manually manage music and videos” checkbox.
Hit apply and try again, you should be able to do the copy now.