Validate after checkout the discount code is valid

validate_discount_code.php

<?php
/**
 * Add a custom validation to the checkout page.
 *
 * @param bool $pmpro_continue_registration
 */
function my_custom_validations( $pmpro_continue_registration ) {
	if ( $pmpro_continue_registration ) {
			// Check if the user has the level needed.
		if ( ! empty( $_REQUEST['discount_code'] ) && $_REQUEST['discount_code'] == 'F37910BEB5' && ! pmpro_hasMembershipLevel( '6' ) ) {
			pmpro_setMessage( __( "Invalid discount code.", 'paid-memberships-pro' ), "pmpro_error" );
			return false;
		}
		
	}
	return $pmpro_continue_registration;
}
add_action( 'pmpro_registration_checks', 'my_custom_validations' );