You can delete a project, but you have to archive it first.
Deleting a project is usually not something you want to do since projects can contain vast amounts of important data. To avoid the risk of losing this data by accident, it would be better to just archive them.
However, if you really want to delete a project, archive it first.
If you ever change your mind you can restore it back.
After you archive it, the sidebar menu on the Project tab allows you to delete the project for good.