Hey @Eros95,
did you follow the redeeming-instructions in this article? In the bottom of that article you can also find some rules, so be sure to check those as well to make sure the error you receive is not because you don't fulfill the rules about redeeming a code.
Apart from that I would also recommend to check if there is any hint in the offer you received with your phone-purchase, just to check if the redeeming needs to be done indeed on Stadia or if you need to redeem it somewhere in the phone itself (I don't know).
You can also contact the Stadia support if you need more assistance in this matter.
