Action items are units of non-task work that you assign to yourself or to others, and that others assign to you.
When you create an action item, you become the action item's owner.
As the owner, you can modify and delete it.
Use the Organizer: Action Items page or the Action Item Details page to delete action items.
At the moment only action items you created can be deleted.
If you were assigned an action item, you can acknowledge it, and then delete the its notification.
An enhancement request to delete other's action items have been submitted: CLRT-16402.
The issue will be corrected on version: 12.2.0