Can I create a book on Leanpub and sell it on another platform?

Yes, you can, because Leanpub does not have any such restrictions. However, please note that other platforms may have their own restrictions.

Here is an article you may find helpful if you're considering using Leanpub: http://help.leanpub.com/author-help/tips-for-self-publishing-and-selling-books-using-leanpub.