When you create a Shopify account, you can prepare/create your store behind the scenes on our 14-day free trial. While you might have connected to Paypal correctly in your Shopify admin, you are not able to view the checkout page while on a trial. To activate the checkout (to accept orders or to view the layout), you will need to select a monthly subscription. Plans and features can be found here if you would like to take a look.
More information can be found here.