Can someone please explain why I'm unable to place a stance mood on the Sun&Moon double sword?
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Because it's not Dual Swords but Dual Nikanas, unique weapon type, the only compatible stance is Mountain's Edge: https://warframe.fandom.com/wiki/Mountain%27s_Edge
It was sellable for some time, so if you dont have it in your equipment, you need to contact the Warframe Support for manual restoration of this mod to your account.