The ISBN (International Standard Book Number) is a unique commercial book identifier. It is a 10 or 13 digit number that is used to identify a specific book, and allows booksellers and libraries to keep track of books.
If your book does not have an ISBN number, you can still sell it on Amazon, but you will need to create a new product listing. To do this, go to Sell on Amazon, and then select Add a Product. From there, you will be able to enter the product details for your book.