Unexpected blank pages can occur due to two reasons: your book is set to start new chapters on a specific side of the page, or there are invisible trailing spaces at the end of a chapter. To fix the issues:
For completely blank pages, click the three dots beside the "Body" title and choose "Begin on > Left side" and then choose "Either side."
For pages with headers or footers on an otherwise blank page, look for trailing spaces. Go to the last sentence and press the down arrow to check for invisible lines and delete them.