Dear Sir, Madam,
I have setup done my first Shopify store, but I do not know how to add a PayPal sandbox account to test checkout. Can you please tell me how I can test payment via Paypal?
You'll have to setup an account at "www.sandbox.paypal.com"
Yes, I have a sandbox account, but when I select: Using: PayPal Express Checkout and enter pay Paypal sandbox, It shows me a form to create new PayPal account.
The same happens to me, I'm fairly certain this happens because it's a test account on the live version of Paypal.