LR Classic Book Module: Hardcover Dust Jacket Won't Work!!

Hi all, hoping someone can help me figure out what I'm doing wrong re the Dust Jacket cover option. I've built an extensive book and originally had chosen the wrap for the cover but decided to change to the Dust Jacket. But for some reason, LR will not let me add photos to the Dust Jacket. I've chosen the first template in the drop down menu and I see the photo cells, but no matter what I do, LR will not let me drag and drop photos into the cells. Weirdly, if I drag a photo to the space between the cell and, say, the spine, the photo will install there, but only in full, 2-page spread mode. Also, as someone else noted back in 2019, for some reason, the Dust Jacket background is gray and has no grid lines, even though my background color is white and I have grid lines turned on. I have the latest OS for Mac and the latest LR Classic installed.

Please see screenshots. Many thanks for any insights, advice, fixes!!