Microsoft 365 features that help users manage their subscriptions, account settings, and billing information.
Did you use the instruction here to redeem the code, so that it can be added to your Microsoft account?
Or, are you trying to use this to renew an existing Microsoft 365 personal plan? If so, you cannot if they were not purchased from the same source, for example, if your plan was originally bought through Microsoft, but now you are trying to renew with code purchased from third party.