Can't put music on iPhone 4s from pc?

I've put music on my iPhone from this computer before. I've had the phone for about 2 months now, but it's always worked for me before. I try to drag a song (and I've tried this with many songs) from my iTunes library to my iPhone music library and it looks like it's going to work, but then the red x symbol comes up each time. The computer is already authorized in iTunes and when I try to click the box which allows me to manage music syncing myself, it says that I have to erase my current library if I want to sync on this computer. If anyone has any solutions, please help!

Should be an option to do a reset on your phone after it's plugged into the computer.