Follow the instructions and section links below:
Afterpay is available for Shopify merchants with a store address in the USA only. The same restriction applies to Cash App Pay.
The store address must correspond to the Afterpay account region.
Do the following:
Go to Shopify Admin > Settings > General.
Under Store Address, the Country must be United States.

The store currency must correspond to the Afterpay account region.
To set the store currency:
Go to: Shopify Admin > Settings > General.
Under Standards and formats, the Currency must be United States Dollars (USD).

Check if you have Automatically at checkout or Automatically when order is fulfilled enabled.

In the Payment Settings, you can also apply manual capture for all payment types. If you have enabled Manually, follow these instructions:
Go to the Add Afterpay at Checkout page and add Afterpay to your checkout pages.