I am unable to sync new iPhone 15 to old iTunes library

What do you mean by old iTunes library? Do you mean that you have installed an older version of iTunes that is unable to work with a newer device running iOS 17? If so is there a reason you're not running the current 12.13.2.3 release of iTunes? iTunes libraries are forward compatible so the latest version of iTunes should be able to open your library.

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See: Sync music between your Windows device and iPhone, iPad, or iPod - Apple Support

And: Sync music between your Mac and iPhone, iPad, or iPod touch - Apple Support