When you design a print cover in the Custom Creator, you'll see a white rectangular box in the lower right corner of the back cover. This is a placeholder for where the barcode will go when you send it to a printer. The white box does not appear in your saved project or on your final download.
If you want to add your own barcode, you can upload that into My Images in your account and then place it in the workspace. It will slide behind that white box.
Learn more about the Bar Code Box in this short YouTube video: Watch Now