Can someone please help me?
I bought witcher 3 on steam and today I wanted to buy a dlc for it. i bought them on kinguin and got a code to use on gog. The problem is that it does not detect the game even though it is installed. I have no idea how to fix it

You need to own the base game here to be able to purchase the DLC here. It's probably the same with Steam.

Kinguin is one of those shady key reseller sites. Keys may or may not be from a legitimate source. It could result in the title being removed from your library.

I suggest you try to get a refund at Kinguin.

tbf Kinguin does clearly state which platform the key(s) are for. That's on the OP for not noticing.

The game and the DLCs have to be on the same platform. Your The Witcher 3 is on Steam and your DLC is from GOG. They will not work. If you go to McDonald's and order a burger, then you go to Burger King and order fries and a coke, you will not get a meal discount -- these products need to be purchased from the same store for this to work.

Although... I do not remember buying The Witcher 3 on GOG, but I have it in my library after linking Steam account to my GOG Galaxy 2.0. You could try to link your Steam account to GOG Galaxy 2.0 and see if you get a copy of TW3 from GOG. If you do, then the GOG key could possibly work. Not sure how long it will take either. I just noticed one day I have a copy of it on GOG, too.

But as the key is from Kinguin, there's no guarantees. It could be a legitimate key someone purchased from sales and is selling it. Could be a money laundering operation with stolen credit cards, who knows. You could try to refund, but I am not sure if those key sites take the keys back. Especially when they cannot be sure if you already used the key. Usually those sites do indicate which platform the key is for, so next time if you use those CD key shops, make sure to double check the platform.