Alert says "iTunes can not be opened while it is being updated." but nothing ever changes
This thread talks about checking for OS updates, and also trying to start the computer in safe mode:
https://discussions.apple.com/thread/7416168?start=0&amp;tstart=0

If you don't have any OS updates to do, you might as well just download a new iTunes app and install a fresh copy:
https://www.apple.com/itunes/download/