Hi, I'm Diane and I'm happy to help you today.

Did they give you instructions on how to download it? If you go to office.com/setup and sign in with my Microsoft account then enter your key, is it accepted? If yes, its a valid retail key and you can download it from your Microsoft account.

Let me know how it goes, I'm here to help you further if needed.

-- Diane

Hello HP P,

Welcome to Microsoft Community.

I am a Microsoft user just like you, I hope you are doing well.

Sorry for the inconvenience. First, I will like you to verify that this Office 2021 license is associated with your Microsoft account.

Go to the Manage Subscription page (https://account.microsoft.com/services) and confirm that your Office 2021 license is associated with your Microsoft account.

Once confirmed, I will suggest you completely uninstall Office from your PC using the Uninstall support tool, then, try installing Office 2021.

To uninstall Office completely use the uninstall support tool

1. Download the Office uninstall support tool.

https://aka.ms/SaRA-officeUninstallFromPC

2. Open the download on your browser, and when you're prompted in the Application Install window, select Install, and then I agree for the Microsoft Services Agreement. Run the file after downloading.
3. From the Uninstall Office products window, select the version of Office you want to uninstall, and then select Next.
4. Follow the prompts on the remaining screens and when prompted, restart your computer. After you restart your computer, the uninstall tool automatically re-opens to complete the final step of the uninstall process. Follow the remaining prompts.

Find link attached from Microsoft on how to uninstall Office a PC:

https://support.microsoft.com/office/9dd49b83-264a-477a-8fcc-2fdf5dbf61d8

To install Office 2021 on new PC:

1. Go to www.office.com and if you're not already signed in, select Sign in.
2. From the Office home page select Install Office.
3. Select Install (or depending on your version, Install Office>).
4. This completes the download of Office to your device. You can now proceed to install Office 2021.
5. Once you're done installing Office 2021, you can then sign in your Microsoft account associated with this license to activate Office apps.

https://support.microsoft.com/office/4414eaaf-0478-48be-9c42-23adc4716658

Please let me know if you need more assistance.

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Warm Regards