沃尔玛api python返回奇怪的错误，不确定我做错了什么

Question

我正在尝试打出Walmart api的请求，并按照文档中所述获取auth令牌：

https://developer.walmart.com/#/apicenter/marketPlace/latest#apiAuthentication

这是我的代码：

import base64
import requests
import json

url = "https://marketplace.walmartapis.com/v3/token"
clientId = ""
clientSecret = ""

auth = clientId+":"+clientSecret
encoded = base64.b64encode(bytes(auth.encode()))
headers = {
    "Authorization": encoded,
    "Content-Type": "application/x-www-form-urlencoded",
    "Accept": "application/json",
    "WM_SVC.NAME": "Walmart Marketplace",
    "WM_QOS.CORRELATION_ID": "123456abcdef",
    "WM_SVC.VERSION": "1.0.0"
}

body ={"grant_type":"client_credentials"}
response=requests.post(url, headers=headers, data=json.dumps(body))
print(response)

我得到：

b'{"error":[{"code":"SYSTEM_ERROR.GMP_GATEWAY_API","info":"System encountered some internal error.","severity":"ERROR","category":"DATA","causes":[],"errorIdentifiers":{}}]}'

我不确定我做错了什么。