The International Standard Book Number (ISBN) is a unique numeric commercial book identifier, and is a global standard in the print publishing industry. Its original role is to help publishers track the sales. However, there is no legal obligation for your ebook to have one: most stores will assign an identifier to your book.
You don't need an ISBN to publish an ebook through PublishDrive.
We don't assign any ISBNs to publishers, but publishers can receive PublishDrive’s Unique Identifier (PUI) which is accepted by most retailers.
If you already have an ISBN assigned to your book, you can carry it on and use it in PublishDrive.
If you have an ISBN for the print version of your book, that can only be used for print distribution. You cannot use the ISBN you received for the print version when distributing your ebook: you need to apply for a new ISBN or distribute your ebook without ISBN.