Hi, Johny,
Many users have reported the problem of being unable to delete projects on El Capitan. (Don't know about Sierra.) No global solution has yet been presented, although several people have reported that one procedure or another has worked for them. I have heard that it is a bug that apple is working on, but that is anecdotal.
You could try removing the media from the project first, and then try deleting it. That has worked for some.
Another successful workaround for some is to create a new library, move the project to the new library, and then try deleting it from there -- or, if that doesn't work, deleting the whole new library with the project in it.
Be sure you are following the regular procedure for deleting projects. That is, open the project browser, move your cursor over the project, select the little circle that appears to the right of the title, and then select Delete Project from the drop down menu.
Hope this helps,
-- Rich