Pick up to three categories where books like yours already sell, and fill all seven keyword boxes with phrases readers would type: genre, setting, character type, mood. Don't repeat words from your title, and never use other authors' names or brands.
Find real phrases
Start typing in Amazon's search box and note the suggestions. Those are searches real readers make.
KDP's rules change from time to time. Check KDP's own help pages for the current requirements before you publish.