I had the same problem, and a co-worker figured out what I was doing wrong.

After you set the IsDestinationPathVariable to True, your variable name goes directly into next property DestinationVariable.

Do NOT (as you might in some other types of tasks) open the Expressions dialog and set it there. If you do, it will treat your destination file name as the variable name.

See in the error message where it says it can't lock the variable 'C:\...' ? That's not a variable name, it's the value of it.