You are unable to delete your project because of possible below reasons:
1. Project Backer’s: When the project is funded even by a single backer.
2. Member Level Settings: ‘Allow Deletion of Projects?’ is disabled for the member belonging to the particular member level.
If Project Owner still wants to delete the project then he can contact to the site admin. Site admin can take proper action in such scenario like: to refund the backed amount to the respective backer and delete the project.
[Note: In case, no one has backed the project or the project is in draft mode, then Project Owner can delete that project.]