close. it should say "the Harry Potter movie series."

Yes, but is that what you want to say? That would mean "movies of the Harry Potter series", but then "Harry Potter movies" is an easier way to say much the same. Or perhaps you mean "Harry Potter movie series", i.e. the series of Harry Potter movies"?

Not quite - it should be the other way round : 'the Harry Potter movie series' Note that 'movie' is singular here, because 'movie series' is a compound noun, and - as I'm sure you've learnt - the first noun in a compound noun has to be singular.