If it ain't broke, break it. MS Word 2016

Can't edit document created in Microsoft word.  Take a great product and make it less great.

https://support.office.com/en-us/article/My-document-title-says-Compatibility-Mode-e9c21150-dbd7-41ea-bb00-f86a38c6a9f1

Cause: You opened a document that was created in an earlier version of Word.