Twenty Sixteen Theme- Having Menu button Permanent

You need to do a couple of things:
– Dequeue the theme’s functions.js file and enqueue your own (so that you can modify it)
– Override the parent theme styles that control the theme’s navigation menu.

First set up a Child Theme: https://codex.wordpress.org/Child_Themes

Let’s start with dequeuing the parent theme’s functions.js file: https://codex.wordpress.org/Function_Reference/wp_dequeue_script

The handle is:

twentysixteen-script

Open up your Child Theme functions.php file and write this line of PHP:

wp_dequeue_script( 'twentysixteen-script' );

Make a copy of the theme’s functions.js file (in the “js” folder) and put that copy inside of your Child Theme folder. Let’s not create a “js” folder inside your Child Theme folder.

Then in your Child Theme functions.php file, add these lines of code:

function wpdocs_theme_name_scripts() {
    wp_enqueue_script( 'twentysixteen-child-script', get_stylesheet_directory_uri() . '/functions.js', array(), '1.0.0', true );
}
add_action( 'wp_enqueue_scripts', 'wpdocs_theme_name_scripts' )

https://developer.wordpress.org/reference/functions/wp_enqueue_script/

And then you can modify your Child Theme functions.js file.

You want to change this:

if ( window.innerWidth < 910 ) {
			if ( menuToggle.hasClass( 'toggled-on' ) ) {
				menuToggle.attr( 'aria-expanded', 'true' );
			} else {
				menuToggle.attr( 'aria-expanded', 'false' );
			}

			if ( siteHeaderMenu.hasClass( 'toggled-on' ) ) {
				siteNavigation.attr( 'aria-expanded', 'true' );
				socialNavigation.attr( 'aria-expanded', 'true' );
			} else {
				siteNavigation.attr( 'aria-expanded', 'false' );
				socialNavigation.attr( 'aria-expanded', 'false' );
			}

			menuToggle.attr( 'aria-controls', 'site-navigation social-navigation' );
		} else {
			menuToggle.removeAttr( 'aria-expanded' );
			siteNavigation.removeAttr( 'aria-expanded' );
			socialNavigation.removeAttr( 'aria-expanded' );
			menuToggle.removeAttr( 'aria-controls' );
		}

To this:

if ( menuToggle.hasClass( 'toggled-on' ) ) {
				menuToggle.attr( 'aria-expanded', 'true' );
			} else {
				menuToggle.attr( 'aria-expanded', 'false' );
			}

			if ( siteHeaderMenu.hasClass( 'toggled-on' ) ) {
				siteNavigation.attr( 'aria-expanded', 'true' );
				socialNavigation.attr( 'aria-expanded', 'true' );
			} else {
				siteNavigation.attr( 'aria-expanded', 'false' );
				socialNavigation.attr( 'aria-expanded', 'false' );
			}

			menuToggle.attr( 'aria-controls', 'site-navigation social-navigation' );

Now in your Child Theme style.css file, add this CSS:

.menu-toggle, .top-menu-toggle {
    display: block;
}

.main-navigation ul {
    display: none;
}