Why do you think it should know what "i" is at that point in your code? You never said what i is, so it's undefined.
I think you want something like
fpbackward = y(2:end) - y(1:end-1)/h;
(but I don't think you even want that, but whatever...that's what you wrote). That will take the prior elements and divide them by the scalar h and subtract them from the latter elements.
