From my Calculus textbook:
Usually the text is careful to make sure the description above a formula establishes all the necessary preconditions to exist for the formula to be true. I noticed here though that $f^ {(n+1)} $ is referred to, without any mention of a requirement that it exist (f being order n differentiable by itself doesn't imply n+1 differentiability). Does the continuity of all the earlier derivatives imply it or is there an unstated assumption?

The statement says
so that next derivative exists