I never said it's a problem. I asked what's the problem that "You can’t quite run it on [...] say, a guitar."

You specifically refer to guitarists’ problem with notation, not the mathematical non-uniqueness. You ask you want to know why “musical notation is a problem for guitarists.”

It’s not.

Am I missing something, or are you directly contradicting your original comment?

https://news.ycombinator.com/item?id=28725817

If you actually read my comment (try it!) you will see that I make a statement about sheet music not uniquely specifying where on a guitar a specific note is to be played. I make no statement whatsoever about guitarists.

You seem to have trouble with the concept that the vast majority of instruments does not yield a recognizable melody simply by pressing the right button at the right time. All of those musicians are doing just fine, thank you very much.