As Leon replied, in case the key has been redeemed on Office.com/setup and Microsoft365.com/setup, you would get this error when redeeming Office again.
Please make sure the Microsoft account that you bound with Office is not lost. Then sign in to your Services & subscriptions page, then install the Office.
Besides, if you want to check the usage of this key, you may contact Microsoft to have a confirmation via phone or online chat.