Do you know which genre your book fits best? This is not always as simple as it sounds. There are sub-genres as well, which help identify your book’s content. Yours may be a combination of two or more. Some evolve over time, like sci-fa (science fiction-fantasy) or sci-fi romance, while in some cases your book may be so unique it doesn’t fit anywhere. Some of the most original and clever stories I’ve read fit that category and I don’t envy the author trying to figure it out. This also makes it very difficult when you have to categorize it on sales sites or during the marketing phase.
Most readers have a favorite genre, so if you want to target your marketing efforts to those most likely to enjoy your book, then you need to figure it out. If it doesn’t fit one of the existing ones, then figure out the closest combination.