generally,when we optimize your website with PageSpeed Insights while you will see missing <title> in result.If you use wordpress you can solve this problem by going to do as below:

  • method 1: add_theme_support( ‘title-tag’ ) to functions.php
    in the theme setup function,you will see:

    add_action( 'after_setup_theme', 'theme_setup' );

function theme_setup() {

add_theme_support( 'title-tag' );

}
  • method 2: going to header.php and add this in <head> 
    <title><?php wp_title( '|', true, 'right' ).bloginfo( 'name' ); ?></title>

    when you add this,it is going to improve your SEO and Accessibility in your website.