When pressing on get silver I get this message same thing when I click on witch queen deluxe. Searching on reddit shows this has been a problem for a few weeks now. Has yet to be officially acknowledged by Bungie from what I can see
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Instead of clicking links in game, just go directly to your platform's online store and purchase them directly. In-game all those links do is re-direct to your platform's online store. And for whatever reason, the redirects don't appear to be working for some players. But if you just go straight to your platform store outside the game you should be able to purchase Silver and D2 DLC's.