I mean with a preposition like "in" that should be dative. And Himmel is masculine, which would change the article to "dem" and not "den". Seeing the article I assumed that perhaps Himmel is a word that when you pluralize it it is the same, so I wrote "the skies" and got it wrong. So I think this is a mistake on Duolingo's end, so I ticketed it.
No, it's fine. "in den Himmel reichen" implies motion (the mountain is personified). That's why it's accusative, not dative.
definite article -> weak inflection