Q: My music from iTunes won't transfer to my iPhone

I just put like 600 songs onto my iTunes to transfer onto my iPhone6s. My iPhone has plenty of space and the music went onto my iTunes with no trouble at all. But every time I try to take the music off iTunes and put onto my iPhone it makes it like it isn't even an option.

Seriously I've been trying to figure this out for 3 days and I have no idea.