Cannot complete authentication to download Word

Since you have tried to uninstall, make sure that you uninstalled Office completely. Full instructions can be found at https://support.microsoft.com/en-us/office/uninstall-office-for-mac-eefa1199-5b58-43af-8a3d-b73dc1a8cae3 .

Try the following steps on macOS to resolve the stalled authentication and reinstall Word.

1. Remove existing Office apps and data
   1. Quit all Office apps.
   2. Open Finder > Applications.
   3. Select Word (and other Microsoft 365 apps if present), then Move to Trash.
   4. In Finder, on the menu bar select Go > Go to Folder…, enter: ~/Library/Containers/ and delete folders starting with:
      • com.microsoft.Word
      • com.microsoft.Excel
      • com.microsoft.Powerpoint
      • com.microsoft.Outlook
      • com.microsoft.OneNote
   5. Go back to ~/Library and open Group Containers. Move these (if present) to Trash:
      • UBF8T346G9.ms
      • UBF8T346G9.Office
   6. Empty the Trash.
2. Fix possible download/authentication issues
   1. Confirm the Mac has a working internet connection and pause other large downloads.
   2. If a previous installer download is stuck, delete the installer file from Downloads, empty Trash, then download again.
   3. Use Safari for the download. In Safari, go to Preferences > Security and ensure Enable Javascript is checked.
   4. In Safari, go to Preferences > Privacy > Manage Website Data… > Remove All, then try the download again so the sign-in/authentication page loads fresh.
3. Reinstall Office/Word
   1. Sign in with the Microsoft account that has the Microsoft 365 or Office license at the account services page (as indicated in the documentation) and start the Office for Mac download again.
   2. If using a perpetual (one-time purchase) Office for Mac product, download the appropriate installer package (for example, Office 2021 for Mac Installer) from the official Microsoft links, then open the package and complete installation.
   3. After installation, open Word and sign in with the same Microsoft account when prompted to activate.

If the authentication window still stalls after clearing Safari website data and retrying, repeat the removal of Office-related app data (Containers and Group Containers) and then reinstall using the appropriate Office for Mac installer.

References:

• What to try if you can't install or activate Office for Mac
• Microsoft Office activation and the Mac App Store
• Microsoft Office apps for Mac quit unexpectedly - Microsoft Q&A