If you select a "slim" jewel case, there is no back cover or tray card. The only place to add your barcode to, would be the front cover.
If you really wanted a tray card, you can "edit" your quote and change the packaging option to a standard jewel case. The tray card will be included with this option.
To edit your quote:
Log into My Account and click on Continue project
on the left side of the page, click on "Edit Quote"
Walk through the online pricing tool to change your packaging
Resave and continue.
If you've selected a different type of packaging which "should" have a back cover, but the artwork displayed to choose that panel is not correct, you may have your artwork set up in the template incorrectly. If you go back to the template, you'll see that it is labeled for each panel. Confirm that your back cover is in the right place.