Another possibility is that your document has editing restrictions applied and you're trying to add the equation to a restricted area.

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]

If you are desperate you could try going old school. The original equation field still works in word 2016 even though it has been deprecated (I know because I use it a lot).

You will need to look at how the various switches work though.

https://jay-freedman.info/EQ%20field%20switches.htm

I quite like it as a tool as it doesn't italicise text like the equation editor 3 used to do but does allow you to apply font effects if you want to.

I'm hoping I can hold out using the old equation field until equations using latex is a bit more established.

My file is in compatibility mode. I have solved the problem by converting the file. "File-Convert" works.