Dear Stephen Bowen,
Good day! Thank you for posting to Microsoft Community. We are happy to help you.
As per your description, please don’t worry, we can work together to narrow down and resolve the situation.
First may I know does your Office 2021 is activated. Does your Office 2021 have product key? if yes, did you redeem your Office key in Microsoft account. If not, please follow the step in this article Where to enter your Office product key, Redeem your product key to redeem your product key in Microsoft account. Then try to sign-in with your associated Microsoft account to check the outcomes.
Meanwhile, could you please upload a screenshot of "when we open any Office product we are asked to sign in to our Microsoft Account." for your reference so we can understand your exact scenario and provide our suggestion in a productive manner, perhaps some related screenshots will be helpful.
Your understanding and patience will be highly appreciated. I hope that you are keeping safe and well!
Sincerely,
Waqas Muhammad
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