I think perhaps you're confusing iTunes the software with the iTunes Store where you buy things. This may help:
You can use iTunes to load any music your iPhone can play; there's no need to buy your music again, though the iTunes Store will be available to you should you ever wish to buy tracks or other content (you can buy directly from your iPhone as well).
Has this been resolved for you?
Have you tried:
--making sure you have the most recent version of itunes.
--making sure your music is in your itunes library?
--if all that is done, make yourself a "new playlist" and add all the music you want on your phone.
--download the playlist to your phone.
If you want all your music on your phone then connect your phone to your computer, when itunes launches, click on your device in the rightside menu, go to music and sync. All or most of your music sould show up on your phone.
-- I am having an issue with getting ALL my music downloaded, so if you are successful and have any tips, please share!