Not sure if you were able to get it resolved. I wouldn't try to update iTunes via App Store. Best way is to: open iTunes> click "iTunes" in the menu> select "Check for Updates…" from drop-down menu. Current version is: 12.6.1.25
You'll either get a window prompting you already have the current version, or option to download, but that download shouldn't be from the App Store. If you do need to update, and within iTunes it stalls and doesn't start or finish the download, you can download the dmg file from Apple's iTunes website here: https://www.apple.com/itunes/download/
Hope this helps.
In God's Harmony