When signing into Office365, sometimes apps will not fully register, and you will see a yellow bar across the top under the ribbon.
This yellow ribbon could have different messages in it related to your account.
If the ribbon says "View Only Your account email@example.com doesn't allow editing on a Mac. To learn more, contact your admin about your Office plan." - most likely the account hasn't been activated.
To Activate your Office account:
1. Open any of the Office apps, such as Word, Excel or PowerPoint
2. Choose the application name from the menu bar - next to the Menu. (It will say Word, or Excel or PowerPower)
3. From the dropdown choose Activate
You may need to sign back into your Office account, but once that's done the yellow bar will go away.