error retrieving auth token

[GRuuuuu]

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.
#216 (comment)
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