I appreciate you getting in touch about this, as it gives us a chance to explain how things work.
Extended free trial codes (like yours, for 60 days) that come with a device purchase (by the way, congrats on adding a new one to your audio system) are meant as a trial for new Roon customers. This means that these codes can only be redeemed on a new Roon accout or during a 14 day free trial.
If an account already has an active subscription, the code cannot be redeemed on that account. If you want to redeem the code on a new account (maybe offer it as a ?), this is where you can enter it: