Yes - you can list an unfinished product, although please be aware that most retailers may want a fully-formed product before they get in contact with you.
If you have all of the necessary information available for the mandatory fields marked by a red asterisk (*), but your product is yet to be manufactured or is not ready to be sold as of yet, there is an option in Section 4 (Distribution & Logistics) where you can select a future date of when your product will be available from. This way you can still list products that have yet to be finalised for buyers to discover.
If you need to go back and edit any parts of your listing that you have previously entered on an unfinished product, you can do this at any time from the 'Products' section of your dashboard. You can find out more on how to edit product listings here.