The default state of the "Synchronization" (backends) dialog looks like this:

First, the "Local File" backend's label and button are totally wrong there. It's not a synchronization service, it's the default file storage backend, so the label should reflect that, and the "Disable syncing" button should be hidden rather than disabled in that case.

If you want the UI to be more helpful, the label could instead explain where the currently used XML data file is located on the filesystem, and a button (or hyperlink?) to xdg-open (or whatever is the proper way in the flatpak portal-enabled world these days) the file browser to its containing location.

Then, if you don't have "python-caldav" installed/available (as is the case on Fedora, for example), if you click the "+" button you get this UI:

That combobox is totally useless when it has zero items in it, and so are the labels and buttons shown there. Instead, it would make much more sense to at least show a human-readable explanation that details which dependencies are missing to allow which backends to be available (but then there's also the question of what to do when only some of the backends are unavailable instead of all of them, if we have more than one sync backend in the future).