AWS CognitoIdentityCredentials has credentials, but fails to get values

Question

I'd like to get a secret key and access key ID as a cognito license.

cognitoUser.authenticateUser(authenticationDetails, {
      onSuccess: function (result) {
        const idToken = result.getIdToken().getJwtToken();

        AWS.config.credentials =
        new AWS.CognitoIdentityCredentials({
        IdentityPoolId:
          'ap-northeast-2:7fc9xxxx-xxxx-xxxx-xxxxx-xxxxx',
        Logins: {
          'cognito-idp.ap-northeast-2.amazonaws.com/userPoolid':
          idToken,
        },
    }) 
}

On the Cognito aws Identity Pool page, it is confirmed that the identity browser has become a credential.

However, in getCredentials, "Could not load credentials from CognitoIdentityCredentials" is generated.

AWS.config.getCredentials(function (err) {
    if (err) return console.error(err.stack);
  else console.log('Access key:', AWS.config.credentials.accessKeyId);
})

The AWS.config.credentials console log shows the following:

Problem not found.

Is it right to get the secret key issued through Cognito Identity Credentials?