My customer keeps getting an error- Authentication/Authorization Failed You do not have permissions to make this API call.

I sent a customer a link, and when he goes to pay the bill, he keeps getting a message that says, 

You haven't entered your Paypal API credentials in the settings, which is why you are getting the Authentication Failed error. To resolve the issue, you will need to enter your Paypal API credentials into the Payment wizard. Make sure to hit Finish once you are done to make sure that everything is saved.

You can also check out this guide on how to get the Paypal API credentials: How to Obtain PayPal API Credentials. Regards.

Thanks for responding. So in the username field do I type my PayPal e-mail
login address????? And in the Password do I type my PayPal Password? And
what do I type in the signature line, my name? Sorry to ask so many
questions, but the website is not intuitive to use.
Best Regards,
Ben
...

No, you do not input your name in the signature nor your username in the API username. If you have a Paypal Pro account, Paypal will provide you with the API Username, API Password, and Signature that you need to enter in those fields. To get these details, please follow this guide: How to Obtain PayPal API Credentials.
Regards.