Once you have manually changed the Page Size or Margins of individual pages, you can no longer change all pages using Document Setup.
You could just make the changes you want without using Document Setup.
The Page Size can be set to the wanted dimensions by selecting all pages in the Pages panel and then clicking the Edit page size button in the bottom of the Pages panel.
The Margins can be set to 0 (or whatever fitting value you want) by selecting all pages in the Pages panel and using Layout > Margins and Columns.
If you want to re-enable Document Setup to change your document, you must first give all pages the same Page Size and Margins (and Bleed and Slug). Then enter Document Setup, type in those same values and click OK. Now if you enter Document Setup again, it has regained control of the settings.