We are sorry for the inconvenience caused by this issue.  This is a known issue but a fix has been issues by Microsoft.

1. Ensure you have been assigned the right Office license:
a. Log in to Office 365 portal (portal.office.com)
b. Settings> Office 365> Subscriptions
c. Under Licenses section, make sure you have been assigned the “The latest desktop version of Office”
2. Make sure you are connecting to the Internet.
3. Clear Office credential caches:
a. Quit Office applications and open Finder > Applications > Utilities > Keychain Access
b. Remove the following:
Microsoft office Identities Cache 2
               Microsoft Office Identities Settings 2

4. Sign in to Office and reactivate it with your account:

              a. Go to the Library folder and open Group Containers. Ctrl+ click each of these folders if present, and Move to Trash.




              b. Restart your mac, open Word 2016 for Mac and sign in with your account.

If the issue persists, please provide detailed steps and related screenshots about how you encounter the issue and the message on your mac, so we can better understand your situation and help you. 

URL of article: https://answers.microsoft.com/en-us/msoffice/forum/all/i-suddenly-cant-edit-my-documents-mac/3b2ab013-0995-43f2-9390-19b63174f95d