"This curriculum already has a grade system."

Where: Grade system screen of a curriculum (Progression > a curriculum > Add a grade system), or Trash > Restore a grade system. Owner only.

Why: a curriculum has at most one grade system. It already has one (created meanwhile, for example in another tab), or an old system is being restored while a new one was created in its place.

What to do: reload the screen to see the current system and edit it. To go back to the old system: first move the current one to the trash (Remove the system), then restore the old one from the Trash (30 days).