$wpdb SELECT query errors

I have created a table on a wordpress database, This database table should have 2 columns.

One for postcode and one for a URL

If the postcode is found in the database, redirect to the corresponding URL

I am inserting rows from my plugin but I cannot select from this table.

Select return always error.

The insert that is working this is the action.php`

this is the form with shortcode

<?php

if ( !defined( 'ABSPATH' ) ) exit;

register_activation_hook( __FILE__, "activate_myplugin" );
register_deactivation_hook( __FILE__, "deactivate_myplugin" );


function activate_myplugin() {
    init_db_myplugin();
}


function postcode_form_function() { 
?>
        <form  method="GET" action="<?php echo plugins_url('action.php', __FILE__ ); ?>">
    <label>postcode</label><input type="text" pattern="[0-9]{5}" title="Five digit zip code" />
    <button name="submit">submit</button>
    </form>
<?php
} 

// register shortcode
add_shortcode('postcode_form', 'postcode_form_function'); 
?>

When I try to select from this table I am taking nothing

<?php  require('../../../wp-blog-header.php');

if(isset($_POST['submit']))
{
    $postcode = $_POST['postcode'];
    // search in all table columns
    $query = "SELECT url 
    FROM wp_4_customer
    WHERE $postcode =postcode 
    ";
    $search_result = submit($query);
    
} else {
   echo 'error';
}

// function to connect and execute the query
function submit($query)
{
    global  $wpdb ;
    $search_result = $wpdb->get_results($query);
    foreach($search_result as $row){`enter code here`
        header('Location: '.$row['url']);
    }
}
?>