You say that you choose PayPal Payments Standard, please explain how you applied this and the selections you made in the checkout wizard. I just would like to have a better understanding of the type of checkout you are trying to implement and the steps that have been taken.
If you are working with Payments Standard you will not have the email receipt option because this is a remote checkout. I would suggest using either Payments Pro or just the Express Checkout option. Both of these are local checkouts that will allow you to send out an email receipt.
As for the error you are getting it is telling you that you do not have permission to make the API call. Express Checkout is normally a part of Payments Pro implementations, these rely on API credentials to make the calls to the server. The API credentials should be available to you in your PayPal profile. To add them in you would need to edit the Express Checkout server behaviors that are on the checkout and pp_confirm pages. In these you will have a place to enter the API details.