A family of Microsoft word processing software products for creating web, email, and print documents.
Hello,
Welcome to Microsoft Community and thank you for posting your query.
Please provide more information before we proceed.
1. What happens when you tap/click on undo option? Is it greyed-out?
2. Does this issue persist in all Word document or any specific documents only?
You may try pressing Ctrl + Z from the keyboard and check whether undo works. Also check and verify if Undo button is enable by clicking on the dropdown button next to Redo button.
I hope the above information helps. Let us know if you need further assistance.
Thank you.