User is trying to create a project, but gets an error that the Project ID is already in use. However, there is no project in the list view that has this same ID.
This can happen if a project with this ID already exists, but has a status other than Active. CORE only displays projects with Active status in the list views and drop-downs. So you should choose a different ID for your project. You might also want to check the existing project to see if it might actually be the one you are trying to create. In that case, you can change its status back to Active. Check out this article for details on how to make inactive projects active.