Bought the game (PoEII) on the website, now I can't play on Steam.

Hello everyone.
I bought the game on the website, I got a Steam key. I activated it, and now after I downloaded the game it says that my account does not have access to the game, and I should buy it on Steam.
What exactly is happening? Is there a fix for this problem? I wouldn't like to buy it again.
Thank you in advance!