adding graphics

The only way to lock an added graphic (e.g. shape) or image in Pages is to restrict its Arrange panel Object Placement to Stay on Page, and how you want its text wrap to interact with surrounding text.

Try not locking a graphic (Object Placement : Move with Text) and observe the result as you change the document contents.

Then you have to set Arrange panel : Object Placement : Stay on Page as that is the only configuration that presents the Lock button in the Arrange panel.