It has nothing to do with append. tuple(3, 4) all by itself raises that error.
The reason is that, as the error message says, tuple expects an iterable argument. You can make a tuple of the contents of a single object by passing that single object to tuple. You can't make a tuple of two things by passing them as separate arguments.
Just do (3, 4) to make a tuple, as in your first example. There's no reason not to use that simple syntax for writing a tuple.