The BigCommerce Checkout Type must be Optimized one-page checkout.
To ensure the checkout type is correct:
Navigate to: BigCommerce Control Panel> Advanced Settings > Checkout.
Under Checkout Settings, the Checkout Type must be: Optimized one-page checkout. See picture below.

The Store Country must correspond to the Afterpay account region.
To ensure the store country is correct:
Navigate to: BigCommerce Control Panel > Store Setup > Store Profile.
Under Contact Information, the Store Country must be one of the following:
See the screenshot below:

Afterpay is available for BigCommerce merchants with a store address in these countries:
🇦🇺 Australia 🇳🇿 New Zealand 🇨🇦 Canada