Manually manage music and videos
After clicking "Manually manage music and videos" in Summary of my iPhone devices, I get a message that my iphone library will be synced.
"You might see a message that your device is synced with another library. This means that you've used a different computer to sync your device in the past, and iTunes can't sync without first erasing and replacing all iTunes content on your device."
What on Earth could possibly be the reason for this? I don't want to sync. I want to manually put *one* more song on the iphone.
iPhone SE