The tags can only contain a call to wp_title(). Use the wp_title filter to modify the output

add_theme_support( 'title-tag' ) doesn’t belong in your header template. It belongs in your functions.php file. Usually, it’s best to wrap it in it’s own function and hook it to after_setup_theme in order to allow plugins and child themes to override it later if they need to. So…

function wpse_add_title_support() {
    add_theme_support( 'title-tag' );
}
add_action ( 'after_setup_theme', 'wpse_add_title_support' );

Once you have declared title support, you can remove the <title> tag from header.php altogether and WP will handle putting it in.