Check to make sure that the “Manually manage music and videos” option is turned on:
1 Connect your device to your computer.
2 Open iTunes. …
3 Locate and click on your device.
4 In the left sidebar, click Summary.
5 Scroll to the Options section.
6 Click the box next to “Manually manage music and videos.”
7 Click Apply.
https://support.apple.com/en-us/HT201593
Hello! In this tutorial I will show you how to transfer music from your iTunes library on your Mac to your iPhone. Cheers.
