This is a common issue. Try these steps:

1. Verify if you are signed in to Microsoft Office
2. Make sure you have the necessary permissions to edit the document
3. Check if another user is currently viewing the document in read-only mode
4. Check with the author of the original document for their access previledges
5. Re-download the document and try again
6. If there are any recent updates to the document, try switching to an older version

If these steps do not help, contact your IT department for further assistance.