I'm not able to make the the Out of the Box Project Object Phase attribute to Read Only.
When I go into Project Object--> Phase attribute the Read only check box is blurred(not Editable). This field is configured in Project layout and being used.
What could be the reason ?
Could you please share the attribute ID? For several OOTB attributes, you will not be able to update attribute properties normally. Have you considered locking the attribute using a workflow? Also, what is the use case for making the attribute read-only? I may be able to advise you better if I know the underlying ask for this.
To make an attribute read-only, a default must be selected - at least from Object - Attribute area.
When you indicate 'it is being used' - doesn't that imply that it needs to be updateable?
Potentially it could be moved to a 'pmo secure' page, where only pmo can update it, if that is what your concern is.
Along the lines of what roland.parrotte has said, previously I have moved the 'Stage' attribute to the PMO only 'Settings' page, but with along with the introduction of a calculated field against the project object that would be a concat of the stage attribute value. Because that field is calculated, it is read-only by behaviour and that calculated attribute is what replaces the OOTB 'Stage' attribute on the 'Properties' => 'Project Summary' page.