I bought the game by discs. I installed the game but it's not actually on my account. So I found the key on the disc display's back and typed it (correctly) but it says 'Code already used'. Discs are not fake and the whole display was taped. It's original and never opened. Please help me. Why is my code already used? (sorry for bad english)

Where did you buy the disk? You need to contact the place of purchase.

You are from Turkey. Let me ask you one question: Did you bought the game from D&R shop?

If the answer is yes read this:

erdak:
"i had the same problem but i solved it..
here is the solution (for Turkey and D&R shop but maybe the other people maybe use similar ways):
-send an email to BILKOM about the code issue (bilkom is support team for the game in Turkey; www.bilkom.com.tr; destek@bilkom.com.tr)
-they will reply in a short time and say that you can return the game with this mail (print it out before you go) to the store and change it.
-change the game.
-and new code will be work.. (you can open the box and try the code in the store via gog.com/redeem web site)

-redeem your code
-update
-install free dlc (if you wish)
-play...
-prepare some potion, oil and bomb before fighting :D"

Source: http://www.gog.com/forum/the_witcher_3_wild_hunt/code_already_used_notification_for_a_nonused_code/post6

That you for explaining this but I didn't buy it from D&R. I bought it from Bimeks

O.K. Then you should go to the store and ask for a new and working key at Bimeks.

It's ok. I contacted PC support and they gave me a fresh link :3

Nice! have fun with your game!