Harry’s unable to leave Hogwarts. There could be ways around the restriction, but they add complexity to the solution.
ETA: Which, to be clear, looks completely unworkable on its in-universe merits. The real problem is that Harry’s never heard a full description of the ritual, neither Dumbledore nor Voldemort would give him one, and Voldemort would have stolen the book that contains it.
Harry’s unable to leave Hogwarts. There could be ways around the restriction, but they add complexity to the solution.
ETA: Which, to be clear, looks completely unworkable on its in-universe merits. The real problem is that Harry’s never heard a full description of the ritual, neither Dumbledore nor Voldemort would give him one, and Voldemort would have stolen the book that contains it.