Inserting EndNote 21 citations into a text box doesn't seem to work. Any way this can be fixed?

If you haven't already, verify that you are using Pages v13.2. Then, install Apple's Pages EndNote Plug-in v3.3. Make certain that you have quit Pages and Endnote beforehand. Reboot. Launch Endnote 21 first, then Pages. The Pages Insert menu : EndNote Citation menu item should be enabled.

Try inserting the EndNote 21 citation into the Text box again. If it doesn't work, then you will need to insert that citation into Body text, either in the current or separate Pages document, then cut and paste into the Text box.

Except for one factor. Pages is not a Word clone by any stretch of the imagination and that means it may not play as nice with EndNote 21 as you would like… and has apparently demonstrated this per the Text box annoyance.

You might try the EndNote community to see if any recent Pages users have found a workaround, but that is a slim chance.

The other factor is that the Pages EndNote Plug-in V3.3 is only compatible through Big Sur (11.*) on Apple Silicon, and may not fully support EndNote 21 features. The last version of EndNote that this plug-in was designed for was X9.3 or later creating a big gap in what it may support.

I doubt if Clarify cares about EndNote interactions with Pages and Apple hasn't released a newer Pages EndNote plug-in in years. This is a classical Escher one hand drawing the other problem and I doubt it will get solved by either company.

If you have access to MS Word 16.78 on macOS can you experiment with a Text box in it and achieve your goal using EndNote 21?

And in addition, Apple has this article from 2021: Add citations to your Pages document with the EndNote plug-in - Apple Support