When you go to edit the "eCard - Selections" element on a published donation form, eCard stationeries appear on the form for selection but when you go to select them and save, an error message pops up saying "Error: Your change would have resulted in an invalid published form! ". When you click to get more information, a pop-up tells you that the problem is that "No eCard Stationeries have been selected for display", even though you just selected one and tried to save.
The reason why you are unable to add the ecard elements is because the donation form is published. Adding the ecard elements and then saving the form means all elements will be live and the Ecard Stationary selector cannot be live without stationaries already selected. You will need to unpublish the form, add the ecard elements and edit them, and then re-publish the form.