Hi, Joon Seo!
This is May from Shopify.
Thank you for taking the time to post your question here. I did some digging regarding this issue, and found that Afterpay only accepts email addresses for customer notifications. So, if your customers try to use a phone number, their payment will fail, and they will be redirected to cart to try again.
To avoid this instance in the future, you will need to only allow customers to check out with email as contact method. Check out the steps below on how to set this up!
From your admin, you’ll want to go to your ‘Settings > Checkout’.
Under the ‘Customer contact’ section, select ‘Customers can only check out using email’.
This way, when your customer enters in a phone number, they will not be able to proceed to checkout, hence, preventing them from being redirected to cart when they attempt payment. If you still see the same issue after changing this setting, please let me know. Feel free to reply back to this thread or contact our 24/7 support, and we’ll happily help!