Pages app won’t let me choose thumbnail view and rename a document

Thumbnail view can be enabled/disabled from the View menu whether in a word-processing layout or page layout Pages document.

If the document name is Untitled.pages because you haven't formally saved the content and gave it a name, do so and change the document name in the Save panel.

If you already named it per the first paragraph and decide you want to rename it again, It is best to use the Finder contextual menu Rename, rather than from the title bar in Pages. However, renaming a document in this manner may confuse the versioning process associated with your changes in the previously named Pages document. You will also need to remember the original name of the document for any Time Machine retrieval, if necessary, prior to the renaming event.