I just came here to look for the same topic, having just discovered that I can't create a new Word document on my phone (Lumia 640 with latest W10M Insider Preview). On my other phone, HTC 8X with WP8.1, it worked fine -- and I remember reading here a few months ago the same issue was reported by Insiders, and that it was a glitch.
Through trial and error, I just resolved the issue:
- Open Word.
- In the top right corner, you should see your name. Click it.
- You will now see an Account screen that list your Microsoft Account and an option to "Add account" (below it). In order to restore the ability to create new Word files on your phone you will need to sign out of your Microsoft Account (in Word). To do this, tap on the email address shown that is associated with your account -- it will expand to reveal a "Sign out" option.
- Click Sign Out,and when prompted to "Remove this account?", click "Remove". Don't worry, you'll re-add in a moment.
- You should now see a screen with black background that says "Choose an account". Click "Microsoft account" or "Work or school account" as appropriate. When prompted, Sign in. After a few moments, you will see the Account page and and that the account has been re-added.
- Click "Close".
- To test that it worked, Click "+ New". Then choose "Blank document". After a moment, you should now be editing a new blank document!