Why am I getting a 401 error at checkout on my new ecommerce site?

[mghaemi]

Hi.  I am new to Shopify and have bulit a basic shop but run into error 401 when I tested to buy a product from my shop.  I have the basic plan that am charged $1 for the first three months.  And I have selected checkout.com as my a merchant to handle my credit card purchases.  Do you know what I am doing wrong?