如果您打算如第一行所述使用沙箱，则由于正在调用生产PayPal服务器而发生了错误.

If you are intending to use sandbox as stated in the first line, then the error is happening because you are calling production PayPal servers.

更改 https://api.paypal.com/v1/oauth2/token

至 https://api.sandbox.paypal.com/v1/oauth2/token