Paypal requires a Third Party Permission Username (?), where do I get this?

[Explorer40]

I am installing Paypal on a new website (www.pickleballpaddleworld.com not live on new host) the install requires various credentials which I got from my Paypal account and entered on the install screen and the install was successful, I do get the Paypal button at checkout . But on my first transaction I get an error returned from Paypal of "(10002) 10002 Security error - Security header is not valid". When I contacted Paypay they said I need to provide them with a Third Party Permission Username. I have contacted my website host and they do not provide that information. Does anyone know where I would get a Third Party Permission Username for the Zencart Paypal Express Checkout? Some added info: www.pickleballpaddleworld.com is live on another host which I am moving away from in early June 2018.

[Design75]

Did you use the same account to log in to PayPal as the one used for Zen Cart? That will not work, as you can not make a payment to yourself.