No support of different orientation with in same document in Pages.
You can use only one page orientation for your document.
Pages doesn’t support a mix of pages in portrait and landscape in the same document. Why???
MacBook Pro 14″, macOS 15.0
Pages has never supported more than one orientation in a document, all the way back to Pages '05, v 1.
The "why" is only something the Apple engineers can answer & they don't participate in these user-to-user forums. Leave feedback for the Pages team using the link in the Pages menu.