Thanks for your speedy reply:
I would go all the way through the check out then enter the credit card information after applying the coupon, but when I clicked the sign up button, all that would happen is that the page would refresh.
No user account would be created in WordPress, and no order would be placed in woo-commerce even with a on-hold status, waiting for payment. I would try many times on desktop or mobile with no luck.
Note from developer:
We had to disable ajax checkout.js to make our coupon work. it is not working with ajax. and even with such situation we able to made a transaction with the plugin and it was successful but after that all the tests failed. we are using live apis for faster result as we need to make it work as early as we can. our system is using the latest woocommerce and wp versions. we tried downgrating but no luck with that.
Thanks for any help you can provide.