We are currently not able to add or edit Content Controls in Word Online. I've found some info on this here:
This link says the following about Content Controls: These display in the document as expected in Reading view. In Editing view the content of fields and content controls are displayed but cannot be edited or updated.
I've also found a couple of questions about this:
It's clear that Word Online does not support adding or editing Content Controls, what isn't clear is why and whether Word Online will support it eventually or not.
Are Content Controls ever going to be fully supported (enabling adding and editing) by Word Online? If so, when will that happen?