What’s going wrong with printing my mini booklet?
Although MacOS doesn't comes with a built-in booklet maker, there are a number of ready-made options to avoid doing this manually. 

PDFs can be easily manipulated ("imposed") into booklet spreads, and there are several apps to do this, both on the Mac App Store and on developers' websites. Some are free, some are very cheap.

Adobe Reader contains a booklet setting in its print menu.

You can find a "PDF Service", which will make booklet PDFs and save them to disk here. You will need to install it in your user Library's "PDF Services" folder, which may need creating if not present. Then it will be available in the PDF button of every print dialog. (It's currently set for A3, but there's an option in the script to change the paper size.)

Some Printer drivers have the function built-in, particularly on large-sheet printers.