I\'m trying to learn paypal payment. I have done a simple AngularJS application that use Paypal-Express-Checkout. As it says on the documentation, first of all I have to do
You have to perform your Paypal transaction on the back end, and the message you're seeing is Paypal enforcing that notion. See this article on CORS for more info.
Your angular http call should be sending the basic transaction info to your server, which will then construct an API request for Paypal, handle the response from Paypal, and then convey that information for consumption by the client side.
[edited to add more info about CORS]