Allo there,
This is likely due to it not being a current or even a released expansion. You'd need to buy an expansion that is already out for it to work.
English
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Would I be able to purchase an older expansion, like Shadowkeep, and it would work? Or is it only the current expansion that counts
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You can buy Shadowkeep, Beyond light , Witchqueen or Lightfall, any one of those will allow you to post on the find fireteam section.