I you have opened a project and has left the editor without closing the project, it will still be marked in our database as being opened, and you won't be able to delete it. After 8 minuttes this mark wil be removed and you will be able to delete it.
If this happens to you, just start a new project under a different name or open the project you want to delete and close it. Then go in the file manager and try to delete it again.