The Coupon code (discount code) given by Tada will work only within the official Shopify checkout. The "Automatic Discounts" option will not work if you are using a third-party checkout provider.
If you are facing this problem, it could be because you are using a checkout in your store that is coming from a third-party provider. For example, if you're using CheckoutX, you could use a workaround where you use our"Predefined Discount" option when creating your discounts and adding them within CheckoutX.
We had a customer that asked why is the discount not being applied using Paypal. Kindly note that Paypal is a payment system, the Shopify checkout can use Paypal. As long as you use the regular Shopify check-out, it will be applied.
If you still have any questions about this, submit a ticket here and we will get back to you as soon as we can.
Keywords:
Will Tada conflict with a third-party checkout provider?
Why isn't the Automatic Discounts option working?
Why my customers are not able to use the discount code on my store?