You have a SharePoint document library where the "Title" property is required and you are using an Office 2016 application. When you try to save the document you get an error saying that:
"This file cannot be saved because some properties are missing or invalid. Use the document properties under the File menu to provide the correct property values. Errors for required and invalid properties are marked With a red border. "
But when you go to the properties section under the File menu no property is highlighted as being required. Even though the "Title" property should be highlighted.
This is a defect.
The fix for this issue has been released, Office 365 Current Channel update 16.0.7870.2020