You can't open the application "iTunes.app" because it is being updated.
When trying to install iPhone update, iTunes locked itself out with 'You can't open the application "iTunesapp" because it is being updated.' message.
iMac 21.5″, macOS 10.13
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iTunes displayed a message that an update was needed to update an iPhone. It turned out it was referring to an update to iTunes which put iTunes in an endless loop of being updated when it wasn't. I installed (the same version) iTunes from www.apple.com/itunes and installed the iPhone update therefrom without a hitch (although it was incredibly slow).
