Authorize.net Error

-- url of the page with the problem -- : keilogic.net/index.php/c22nsa-store
-- HikaShop version -- : 4.5.1
-- Joomla version -- : 4.1.2
-- PHP version -- : 7.4.29
-- Browser(s) name and version -- : Chrome 100.0.4896.127
-- Error-message(debug-mod must be tuned on) -- : (14) The referrer, relay response or receipt link URL is invalid.
or
405 - HTTP verb used to access this page is not allowed.

I am using the Authorize.net plugin. When an order is submitted, Authorize.net returns the following error:
(14) The referrer, relay response or receipt link URL is invalid.

I read in an earlier post that "The response URL should be empty in your merchant account." and it is.

If I set up the Authorize.net plugin to use my sandbox account, I get a different error as follows:
405 - HTTP verb used to access this page is not allowed.
The page you are looking for cannot be displayed because an invalid method (HTTP verb) was used to attempt access.

The site is currently configured to use the sandbox.

Any help would be appreciated.