Hi there, I was struggling for two hours with this problem yesterday, I finally found MY solution and thought that it might help you. Because I don't normally play music from my computer, only from my IOS devices, I keep storage drives that hold music and photos off until I need them. When I connected my brand new iPhone X to iTunes, it showed my entire library of music, even though that storage drive wasn't active. Nothing worked until I tried switching to "manually manage music" and dragged a few albums over and that's when the notification popped up that the items couldn't be found. I turned on the drive and all is well. Good luck.