problem with Holiday "calendar"

V. 8.3.1

Hi everybody,
I have a problem with the management of an "out duty" project.
I am tring to add activities like "holiday", "sickness".. etc. into an administrative "out duty" project (as suggested), however I obtain the following error:

can't create an activity on the project dedicated to dedicated to leaves system

So I tried to create a new administrative project, and to add activities to this, and it works.
So I tried to remove the first administrative project, by I obtain the following error:

can't delete the project dedicated to the leaves system

And the record for this project is very poor: I cannot modify anything (see the attached pictures)

while in the second one I have all the fileds available:


What is wrong?