您的“dataBilling”属性（“dataBilling“）中有额外的空间。

const json = `
  {
    "data": [
      {
        "dataBilling ": [
          {
            "amount": 400,
            "edited": false,
            "billingGroup": "CONSULTATIONANDVISIT",
            "qty": 1,
            "discount": "0",
            "grossAmount": 400,
            "excludeFlag": false,
            "billingDescription": "PAID"
          }
        ]
      },
      {
        "dataBilling ": [
          {
            "amount": 427,
            "edited": false,
            "billingGroup": "MEDICINES",
            "qty": 1,
            "discount": "0",
            "grossAmount": 427,
            "excludeFlag": false,
            "billingDescription": "PAID"
          }
        ]
      }
    ]
  }
`;

const obj = JSON.parse(json);

const total = obj.data.reduce((acc, curr) => acc + curr['dataBilling '][0].amount, 0);

console.log(total);

// errors, there is no property "dataBilling"
const total = obj.data.reduce((acc, curr) => acc + curr.dataBilling[0].amount, 0);

您需要在创建对象的源上修复此问题。