The Travels of Sir John Mandeville is best classified in the medieval literature genre. It is probably best described as fiction, although it is written like a non-fiction travel journal or memoir. The work is old, and thus we don't know as much about the author as we know about modern authors. The book is written with accurate geographical descriptions and fantastic embellishments and creatures. Because the work was written in the fourteenth century, it does not make sense to categorize it in one of our modern genres, such as science fiction. The genre of science fiction as we know it today did not really exist at the time The Travels of Sir John Mandeville was written, and even if this book were written today, it would not belong in the science fiction genre.
Science fiction is characterized broadly as fiction that incorporates scientific topics while writing with a rational approach. Although the genre of science fiction was not developed at the time of publication, The Travels of Sir John Mandeville does have some of the hallmarks of today's science fiction, such as fantastic creatures, grand adventure, and travels to unknown realms. However, most of the technologies, ideas, and discoveries that inspire today's science fiction simply were not around to inspire fiction writers of the fourteenth century.
Often, however, there is an overlap between the genres of science fiction and fantasy. Fantasy is a genre of fiction in which the rules in the written universe deviate from reality. Fantasy as a genre is a closer fit for The Travels of Sir John Mandeville, but since it is such an old work, you would still most likely find it in the medieval literature section.