Reply To: Hide header and nav bar

.page-id-123 #masthead {
    display: none;
}

Add the above CSS to Customize > Additional CSS.

Replace 123 part of .page-id-123 with the ID of your actual page.

You can find the page ID when you edit the page you will see /wp-admin/post.php?post=123&action=edit in the address bar. The ‘post=123’ part is the page ID.