This issue normally occurs if you install the Authipay Gateway after the Currency Switcher. One of the features of the Currency Switcher is the possibility of enabling payment gateways depending on the currency. When you install a new gateway, it does not get automatically enabled in the currency switcher, therefore it will not appear on the checkout page.
Please go to WooCommerce > Currency Switcher Options > Payment Gateways and make sure that Authipay is listed amongst the enabled gateways. If it is not listed, simply type its name in the field next to the currencies for which you would like to enable it, and save the settings. Authipay should now appear as one of the checkout options.