The categories that are available are the categories that are commonly available in major podcast directories, like iTunes. The category that you choose affects where your podcast will show up when your listeners search or browse for it in their app of choice.
If you can't find a category (or categories) that line up well with the content of your show, you can try a number of things.
Look at the subject matter that you cover, rather than the overall show topic. A Q&A show might struggle to find a Q&A category, but there might be categories that line up with the individual questions that are commonly asked.
Browse Apple Podcasts and Google Play to see what other podcasts similar to yours use. Even if the categories are not exactly accurate, it is useful for your podcast to be available in the same place as other podcasts with similar user bases.
Try to find categories that are related. Remember that categories are useful for helping potential listeners to find your show. What might a listener be looking for that would make them interested in what you're producing?