Word opening in compatibility mode when it's .docx.

If you install a new version of Office over an old one, it picks up settings / configurations from the old installation. Specifically from the NORMAL.DOT/NORMAL.DOTM file. If you start a version of Word that is newer than the NORMAL.DOTM, you will get the compatibility mode message. You either have to convert the old NORMAL.DOTM into the new format, or rename it and let Word generate a new one.

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! Converting documents to a newer format 365 / 2019 - 2010
https://support.office.com/en-us/article/converting-documents-to-a-newer-format-34ec742e-f0f9-4d95-bbe3-3ee8e30a86fa
Mac: https://support.microsoft.com/en-us/office/my-document-title-says-compatibility-mode-e9c21150-dbd7-41ea-bb00-f86a38c6a9f1 
When you're using a version of Word that's newer than the version used to create your document, you typically see Compatibility Mode in the title bar. Compatibility Mode temporarily disables new or enhanced Word features so that the document can still be edited by people using the earlier version of Word. Compatibility Mode also preserves the layout of the document.
Converting the document to the newer format lets you use newer features, but conversion might create difficulties for people who are using earlier versions of Word to edit the document. If you convert the document, they might not be able to work with certain portions of the document that were created in the newer versions of Word.
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Use Word 2016-2007 to open documents created in earlier versions of Wordhttps://support.office.com/en-gb/article/Use-Word-2016-to-open-documents-created-in-earlier-versions-of-Word-5b38a00a-840b-4719-a8a3-ce155df82554
When you open a document in Word 2016 that was created in Word 2010 or earlier, you see Compatibility Mode in the title bar of the document window. Compatibility Mode temporarily disables new or enhanced Word 2016 features so that people using Word 2010 or earlier will still have full editing capabilities. Compatibility Mode also preserves the layout of the document.
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**Text Boundaries Template for Word 2013 and later** ****
http://addbalance.com/word/download/TextBoundaries2013.zip
Word 2013 changed the text rendering engine in Word so that "text boundaries" are around individual paragraphs rather than displaying the page margins. Many people prefer the old method.This is a zip file containing two letter-size macro- and text-free templates and a read-me document. Documents created using one template will display text boundaries like they were displayed in Word 2003-2010, they will be in compatibility mode. This compatibility mode method does not work in Word 2019/365. Documents created using the second template will display with a pseudo text boundary that is actually a page border set to be one point from text. Those documents will not be in compatibility mode. This is a work-around, not a solution, but may be helpful. See the Read Me file for directions. Version 15.3. For more on the problem this addresses, see Text Boundaries Display in Microsoft Word.

The fact that you can't change the zoom in Print Layout view indicates that the Side to Side option in the Page Movement group has been enabled. The option is intended to mimic the feeling of turning the page in a book (I suppose).

Click the Vertical option to restore the normal behavior of the zoom.