Hello, thank you for reaching out to Microsoft Q&A.
Are you encountering this error during product key redemption, or after the key has already been redeemed and you're installing Office?
Try redeeming your product key using an incognito tab or with a different browser.
https://setup.office.com/redeem/enter-key
Also, please let me know which Office edition your product key belongs to, such as Office Home 2024, Office Professional Plus, or Office LTSC or Office 365 Personal or Family Subscription.