Keep Getting Error: "There are purchased items on the iPhone..."

I want to update the iOS on my iPhone XS. I have, multiple times, gone to iTunes, File>Devices>Transfer Purchases...but it gets to "Preparing to transfer" and just stops and doesn't do anything. When I try to update the iOS...I then continue to get the message, "There are purchased items on the iPhone that have not been transferred to your iTunes library. You should transfer these items to your iTunes library before updating this iPhone. Are you sure you want to continue?"

Ideas??