Looks like you have the “encrypted button only” option enabled in your PayPal account. That will prevent any other transaction other than buttons created inside your Paypal account. So you need to uncheck that option if you want to use any plugin to accept payment.
Please watch the following video which has some explanation:
https://www.youtube.com/watch?v=xl7HMsqCNPQ
Let me know how you go after that.