我已存储此服务帐户密钥（my-key。json）文件在我的下载文件夹（ubuntu），然后我运行这个命令到我的控制台
export GOOGLE_APPLICATION_CREDENTIALS="/home/user/Downloads/my-key.json"
根据google cloud。现在我正在运行这段代码，但它抛出了错误。

const language = require('@google-cloud/language');

const quickstart = async function () {
  // Instantiates a client
  const client = new language.LanguageServiceClient();
 
  // The text to analyze
  const text = 'Hello, world!';
 
  const document = {
    content: text,
    type: 'PLAIN_TEXT',
  };
 
  // Detects the sentiment of the text
  const [result] = await client.analyzeSentiment({document: document});
  const sentiment = result.documentSentiment;
 
  console.log(`Text: ${text}`);
  console.log(`Sentiment score: ${sentiment.score}`);
  console.log(`Sentiment magnitude: ${sentiment.magnitude}`);
}

quickstart();

**ERORR** -
(node:13928) UnhandledPromiseRejectionWarning: Error: Could not load the default credentials. Browse to https://cloud.google.com/docs/authentication/getting-started for more information.
    at GoogleAuth.getApplicationDefaultAsync (/home/hardy/Documents/personal/project/node_modules/google-auth-library/build/src/auth/googleauth.js:154:19)
    at processTicksAndRejections (internal/process/task_queues.js:94:5)
    at async GoogleAuth.getClient (/home/hardy/Documents/personal/project/node_modules/google-auth-library/build/src/auth/googleauth.js:485:17)
    at async GrpcClient._getCredentials (/home/hardy/Documents/personal/project/node_modules/google-gax/build/src/grpc.js:88:24)
    at async GrpcClient.createStub (/home/hardy/Documents/personal/project/node_modules/google-gax/build/src/grpc.js:213:23)