Hey guys! If you are hitting this error we have a whole writeup explaining why that may be occurring here:
tl;dr you may have already had your account bound prior to the update via Steam Economy and may need to contact support.warframe.com depending on the scenario you fall under:
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Scenario A: If I have Steam account A already bound to Warframe account A with Steam Economy, but I go to bind Steam account A to Warframe account B, C, D. etc. after Koumei & The Five Fates launches, I will not be able to, and will receive an error and have to submit a ticket to support. This is because that Steam account is already bound to a Warframe account, and cannot be bound twice.
Scenario B: If I have Steam account A already bound to Warframe account A with Steam Economy, but then I go to bind Steam Account B, C, D etc... to Warframe account A after Koumei & The Five Fates launches, I will not be able to, and will receive an error and have to submit a ticket to support. This is because that Warframe account is already bound to a Steam account, and cannot be bound twice.
Scenario C: If I have Steam account A already bound to Warframe account A through Steam Economy before the update next week, I will NOT have issues with the new account binding system after Koumei & The Five Fates launches as long as I am binding Steam Account A to Warframe account A
Note: Other purchases made from the Steam Store do not require you to bind to Steam Economy, only if you had specifically purchased TennoGen.