Admin-ajax.php 400 error

You have two problems
in PHP you need change this

is bad:

wp_localize_script( 'customjquery', 'ajaxurl', admin_url( 'admin- 
   ajax.php' ) );

change to:

wp_localize_script( 'customjquery', 'my_custom_vars', ['ajax_url' => admin_url('admin-ajax.php')] );

and your javascript need this changes

is bad:

$.ajax({
        type: 'POST', // use $_POST method to submit data
        dataType: "html",
        url: ajaxurl,

change to:

$.ajax({
        type: 'POST', // use $_POST method to submit data
        dataType: "html",
        url: my_custom_vars.ajax_url,