I need to recover the key for Microsoft Office

Once you have redeemed your product key, you can't use it again. The associated account effectively becomes the product key, and you use your account when you need to reinstall Office. Go to https://account.microsoft.com/services and scroll down until you find your Office 2021 product. You should see an Install button. If you run into trouble with activation, you may have to contact Microsoft support.

Because the original product key is lost and the seller is no longer available, the options are limited:

1. If the one-time purchase Office was ever activated using a Microsoft account, sign in with that same account at the Microsoft account dashboard to reinstall Office without needing the original key. The product key is not shown online, but the license is tied to the account.
2. If the key card or email is damaged but valid proof of purchase is available (for example, an invoice or receipt from Amazon), Microsoft support may be able to help replace the key or offer other options. In that case, contact Microsoft support directly.
3. If the key is lost and cannot be recovered, and there is no way to prove purchase in a way Microsoft support accepts, a new Office or Microsoft 365 license must be purchased.

For assistance with a lost or damaged key and proof of purchase, contact Microsoft support: https://support.microsoft.com/contactus/

References:

• Using product keys with Microsoft 365
• Where to enter your Office product key
• How to reinstall Office & Student 2019 - Microsoft Q&A