@M4RHUN I'm having the exact same issue. In my case, my Far Cry 3 account still has my older username and I bought FC5 Gold Edition after changing my username. The game recognizes my current username as a foreign account and therefore the key is invalid because is supposedly already redeemed on my main username.
The only "fix" for this issue would be getting a new Deluxe Edition key from Ubisoft themselves or for them to change your in-game Far Cry 3's username to match your current username.