ipod 6th gen

Because there is not - and has never been - iTunes for an iPod.

iTunes is the application (or programme) for your computer. You use iTunes to copy music from your iTunes Library (the music and other media) to your iPod. The iPod uses the music app to play that music and on a 6th gen Touch, the music app is denoted by a musical note icon:

If you mean that you are trying to copy "music" from your iTunes Library (on your computer) to your iPod, then that is music, not iTunes. If you mean something else, please provide detail of what it is you are referring to and in either case, provide more detail about the steps you use, which step doesn't work as you expect and what happens when you try that step.