just to clarify a bit, in case it’s needed, there’s a dropdown menu associated with the Community item on the top navigation bar on the iNaturalist home page, but you can access the menu only by hovering over the Community item. if you’re using a finger or a non-active stylus on a touchscreen, you can only tap, not hover. so then you can’t get the menu to appear.
that said, there are other links to the forum at the bottom of the home page and the latest forum post preview box, and you can always bookmark the forum page so you can go to it directly.