Trying to build a cleric, and several domains are missing from the drop-down: war; trickery; nature; and some others. The description lists those domains, but they are not available to choose. I wanted a nature domain cleric..... Anyhow it says missing choices can be added in the Marketplace, but all I see there are source books. Anybody know how to build a nature domain cleric?

Nature Domain (and the other two you mentioned) are in the Player's Handbook, which you do have to purchase since they aren't Open Game License.

If you go into the page for the PHB in the marketplace, you can purchase just the subdomain, and also any individual spells you might want (Grasping Vine, for example, also requires purchase).

I noticed this too.  I wanted to build a spirit domain cleric using the 2014 rules, but I can't find spirit domain anywhere.  It didn't even show up when I typed it into the search bar.  My dm owes every book they made for the 2014 rules and enabled sharing for them in our campaign, so there’s no reason it shouldn't be there.

Spirit domain is not a subclass of cleric officially, as far as I know. Its homebrew so would have to access it that way

https://www.dndbeyond.com/homebrew/subclasses?filter-name=Spirit domain&filter-author=&filter-author-previous=&filter-author-symbol=&filter-rating=-40