Hi Barbhaz,
Thank you for reaching out. My name is Leo, a user just like you. I'd be happy to help you with your concern.
May I know if it is affecting all files? Also, could you clarify where you purchased the new Office 2021? Were you successful in activating it on your Mac? Do you see any error code or message? When you create a new Word document, does the issue also occur? For this issue, I recommend trying to remove the license, then attempt to reactivate your Office. To do this, follow the steps below:
"1. Download the license removal tool via https://go.microsoft.com/fwlink/?linkid=849815
2. Open the .pkg file from your Mac's Downloads folder.
3. Select Continue and follow the prompts in the setup wizard.
4. When you're ready to complete the license removal, select Install. You may need to enter your Mac's local admin account password.
After the tool successfully removes licenses, select Close.
5. Now open any Office app and sign in to activate with the email account associated with your Microsoft for Mac license or use the license key if you have.
6. Uninstall the License Removal Tool by dragging the Microsoft_Office_License_Removal file from Finder > Downloads to Trash."
Then check if it resolves the issue. If not, kindly provide more information regarding the issue and a screenshot for us to understand it better. In case the issue still persists, please don't hesitate to let us know. We the community users are always here to help. Thank you!
I pulled the above steps from Official Microsoft Support website. For reference, here's the link below:
Your understanding and patience will be highly appreciated. I hope I provided a helpful information regarding your concern! Let me know if you have any further questions. Be safe always!
Best Regards,
Leo