Pages EndNote Citations Disabled for Shared Documents
After a great deal of working with Apple Support, I found that I could, for some reason, actually insert "EndNote Citation" in a new document with the content from the old pasted in. Neither of us were able to figure out why it wouldn't work with the old. After the call, I did a little more fiddling and found that when I stopped sharing the old document, the "Document Body" option became selectable again (it had been grayed out but checked) and I was able to insert the citations.
My best guess is that this selection is disabled for shared documents because unchecking "Document Body" would delete all of the text by converting the document into a template, but it seems rather broken that this also means I am unable to use the insert "EndNote Citation" functionality. Again, this seems related to the fact that the citations can only be inserted with "Document Body" checked, but it should be doable if the shared document is not a template.
MacBook Pro (M3, 2023)