As long as there is one customer order (COA) that has been created with a link to a project order (POD), you may not delete the project order (POD).

When trying, you will get the following error message: You cannot delete the project order because it belongs to COA/xxxxxx.

If you have a project order (POD) that you are not going to use, you can close it to status 9 with the context menu option "Complete project order"

read more about project orders (POD) here: Project order - POD