admin-ajax.php HTTP400: BAD REQUEST – The request could not be processed by the server due to invalid syntax

You can’t send the data to admin-ajax.php as JSON:

data: JSON.stringify({
    action: "process_coupon"
}),

For the wp_ajax_ actions to be fired, WordPress checks $_REQUEST['action'], but this is not populated by PHP when the form is submitted as JSON. The correct way to send the request with jQuery is to pass the data like this:

data: {
    action: "process_coupon"
},

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