At the moment, it isn't possible to fail objectives or quests. Support for failing objectives seem to exist in the code, while it doesn't yet for quests.

• There could be an effect added to fail an objective, similar to the "Complete objective" one.
• For quests, this requires more work, as quests are marked complete when the step it is on is a final one. This could be achieved by adding a dropdown next to the "Final step" checkbox, disabled when the checkbox is unchecked, and allowing to switch whether the final step will complete or fail the quest. (Default should stay as "complete"). A failed quest with the "Progression" box checked shouldn't add to the progression bar (but it should still affect the total number of quests used for computing the progression score).