Applying coupons using woocommerce rest api
you need to add "coupon_lines" to your order object as stated hereEvery order should have and "coupon_lines" array containing "id","code","amount" for you desired coupon.
https://woocommerce.github.io/woocommerce-rest-api-docs/v3.html#view-customer-orders
'coupon_lines' => [ [ 'id' => 55, 'code' => free50, 'amount' => '10.75', ]]
Can you please add coupons id and after check?
"coupon_lines":[{ 'id' => '', 'code'=>'coupon1', 'amount'=>'1.00'}]