So I want to grab the stellaris anniversary portraits but when I'm at the shop menu, I get this. Do I have to buy something else to get this or what?

If you don't own Stellaris on GOG (or have it in the cart along with the DLC) you won't be able to get any of its DLC (even free ones).

If you DO already have Stellaris in your library under that account, it might be a bug that apparently has been popping up lately. Try checking & unchecking the "Gift this order" box, see if that makes a difference.

Some people have issues claiming free stuff like demos etc from time to time.

https://www.gog.com/forum/general/cant_redeem_free_games

Try this thread for more info. (But like HunchBluntley said, unless you already own Stellaris you won't be able to get it)