Does each white rectangle in your preview represent a page?
If so, your layout is at least a small amount too big to fit on one page and your are getting an auto-matic page break to print the small amount of extra blank space on the second page.
Enter layout mode.
Select the page size and orientation options you'll use when printing in Printer Setup.
Look for a heavy dashed horizontal line showing the position of the page break. (It may merge with the bottom boundary of a layout part and not be easy to see.
Reduce the size of your layout body or other layout part until this line disappears. It could be a size change as small as one pixel.
I assumed that you were recommending that I reduce the size of the layout body by changing the % number in the scale window in Page Setup, so that's what I did. I had to go all the way down to 68% before the dashed horizontal line disappeared, but that did solve the problem. The blank cards disappeared in the preview mode, and the job was printed without problem.
I learned something, and another project is out of the way. Onward and Upward!
You must have a Mac as that approach doesn't work on Windows. What I meant was to enter layout mode and either drag the bottom boundary of the body part up or click on the body part label and use the height box on the inspector's position tab to reduce the height of the layout body.