It looks like you have a typo in your error message; it should be
ERROR: UndefVarError: .. not defined
(You have an extra ..)
Anyway, the error message points to the problem:
return [xs[i]..xs[i+1] for i in 1:length(xs)-1]
uses .., which is undefined. It can be defined by importing ValidatedNumerics.jl, which was imported earlier on in the blog post.