How can I transfer backed up iphone 6 files from Itunes to new iphone XR
You may be experiencing compatibility issues between an older version of macOS and your iPhone XR. (Syncing files using iTunes was deprecated in macOS 10.15, so iOS 13 on your iPhone may reflect this change.)

Make sure your Mac and iPhone have the latest software release installed, then follow this Apple Support article to sync your iPhone 6 backup with your iPhone XR.