iTunes won’t start on my MacBook Air.

See Use safe mode to isolate issues with your Mac - Apple Support and see if you can get iTunes to open in safe mode. You may find it will then open when you start your computer normally.

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Try this again after rebooting:

Try holding command+option as you launch iTunes to see if will open in its own safe mode.

Create a new user account, log into it, see if iTunes opens there.

Try holding down option as you launch iTunes to see it you get prompted to choose or create a new library. If so try creating a new library to test it with, then close iTunes and try selecting your original library.

tt2

Try holding command+option as you launch iTunes to see if will open in its own safe mode.

Update to macOS 10.14.1 Mojave if you haven't already done so.

tt2