Sticky header, also called fixed header, is a smart navigation tool that sticks the header to the top of the screen as the website users scrolls down a page.
So, to enable this option please navigate to WP Dashboard > Wiz > Customizer & Page Options > enable Sticky Header.
Once you’ve enabled it, navigate to WP Dashboard > Appearance > Customize > Layout > Header > Sticky Header and you will find sticky header. For more information about sticky header options please refer to Sticky Header article.