remove_action( 'woocommerce_before_checkout_form', 'woocommerce_checkout_coupon_form');
add_action( 'woocommerce_after_checkout_form', 'woocommerce_checkout_coupon_form' );

https://www.cloudways.com/blog/move-woocommerce-coupon-field/
https://woocommerce.github.io/code-reference/hooks/hooks.html