Problems with Paypal Payments

Hi,

My Paypal app was approved. However, when I try to make a payment in sandbox mode, Paypal gives me the following error message:

“This transaction is invalid. Go back to the website and try again.”

Please, find a screenshot in the attachment. Note, that it is in spanish, but you will get my point.

Besides, when I switch to production mode, the applications does not lead to PayPal and stays in step 2 of the template.

Thanks in advance

Jose