"This category no longer exists: it may have been moved to the trash."

Where: Curriculums screen, when adding an item to a category or moving an item into one.

Why: the chosen category no longer exists in this curriculum: it was moved to the trash while the screen was open, or it belongs to another curriculum. Only the owner manages categories.

What to do: reload the screen to get the current categories, then choose a category that is still there. The owner can restore a category from the Trash for 30 days.