iTunes on iMac won't start up after upgrade (error -10699)

Solution 1:

I had the same problem and was able to solve it by deleting iTunes and reinstalling.

The Finder tries quite hard to stop you from deleting iTunes; instead you can open Terminal.app and 'sudo rm -rf /Applications/iTunes.app'. You can download the replacement copy of iTunes from https://www.apple.com/itunes/download/

Solution 2:

As a temporary solution, you can execute iTunes binary directly by doing /Applications/iTunes.app/Contents/MacOS/iTunes in the terminal.

Code -10699 returned by open -a iTunes indicates some error in Launch Services (similar error codes are documented but the very thing is not). Executing binary in this way somehow sidesteps the problem.