Hi!
In Roseta, I’ve the “Featured Content”-Section in the admin menu. There I created a “featured box” (screenshot box created)
I open: Customizer > Lading Page > Featured Boxes 2
I expected to find the saved featured box in the dropdown field, but the section of “featured boxes” is empty; see screenshot: Screenshot Dropdown featured boxes
Featured boxes are selectable/displayed in bulk, not individually. Ensure your boxes have a category assigned to them and that category will show up in the content selector.