There are some categories that we cannot select but Amazon's system can place you in them.
So best thing is to choose the categories that you are able to select which are most relevant to your book.
If you are unsure of which categories, you can try using our free KDP category tool to find inspiration: https://selfpublishingtitans.com/tools/KDP-Kindle-Category-Finder
Another thing you can do is look at multiple competitors to get an idea of some you might want to also use.
Hope this help and feel free to ask new questions anytime. We will answer them all and help as best we can.
Best,
Corvin
Some of my competitors are in categories that are not in Amazons list,and are more relvant.How do we find them?