error retrieving auth token

[GRuuuuu]

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Hello, Following README, I opened the .env file and add the service credentials. My service instance is using apikey and url as a credentials. so, I fill out like this same as README.

# Environment variables
# You need to provide either username and password
SPEECH_TO_TEXT_USERNAME=
SPEECH_TO_TEXT_PASSWORD=
# OR IAM API key and URL
SPEECH_TO_TEXT_IAM_APIKEY=pbPNV17P2WtnMIIgKtj6QHj89Z9DrBMtsXhWEmmhYRsc
SPEECH_TO_TEXT_IAM_URL=https://stream.watsonplatform.net/speech-to-text/api
# Service URL
SPEECH_TO_TEXT_URL=https://stream.watsonplatform.net/speech-to-text/api

as you see, the service instance's region is dallas (us-south)

I thought everything is ok, but I got error.

Unauthorized: Access is denied due to invalid credentials.
    at RequestWrapper.formatError

After few searches, I found a solution below. https://github.com/watson-developer-cloud/speech-to-text-nodejs/issues/216#issuecomment-458653521 and I solved the problem.

But that solution is just temporary solution. I think source code should be fixed. or, should rewrite README file.

and I am curious about why this happen. can not SPEECH_TO_TEXT_IAM_URL get a token?

Thank you