User Field Prompt allows you to show a user field form in any Discourse post. Changes made in the form will be saved in the current user profile. It’s useful to ask users to make changes to a user field without having to force them to search the field in their profile.
Usage for a user field named age and another one named best fruits:
My bad the TC was deactivated, it’s now active, I created a new topic: User field prompt test 2 - Catégorie test - Dessein-Tech the frame around the field appears but is not editable. On top of that, the delete button and edit button for that topic do not work (there a delay of 30 seconds before the topic is actually deleted or can be edited). The wrench icon does not react too. This happens in Preview mode too.
I think the problem is that your site is running a legacy version of the post-stream, because discourse-nationalflags has not yet been updated for the new post stream. As an admin, you should be seeing a red banner about that issue.
The recent fixes for user-field-prompt were built only with the new post-stream in mind, so that’s why it’s causing errors on your site.
So if you need this user-field feature right now, I’d recommend disabling the nationalflags plugin. Otherwise, you’ll need to wait for the developer of that plugin to make the necessary updates.