How to fix this:
1. On the landing page you're wanting to add to an editorial card, press the "Edit" tab at the top.
2. Expand the "Editorial card fields" area by clicking the blue name.
Here you'll find fields to control the title, description/subtitle/subhed, and the image for the editorial card. Fill these in.
3. When the Editorial Card fields are populated, Save the landing page by pressing the green Save button at the bottom.