Can you share your code that's causing this? The SDK is coded to make the proper request. If you're using the SDK's auth helper, it shouldn't be possible to make a request using an incorrect HTTP method.
os.environ['PURECLOUD_CLIENT_ID'] gets the value of the environment variable PURECLOUD_CLIENT_ID. It looks like you replaced the name of the environment variables with your client id and secret. If you're going to hardcode the values, just use the hardcoded values and do not attempt to retrieve them from environment variables.
You're right, I replaced the name of the environment variables with my client ID & secret ID generated in PureCloud.
How my application will authenticate in my case without replacing the variables ?
I'm not sure what you're talking about. You're writing a Python script, correct? In that context, I don't understand where you're clicking a link for anything.
The tutorial retrieves the client credentials from environment variables. You're welcome to write your app however you like to handle your client credentials. Just remember that the client secret should be handled like a password.
That tutorial is for when you're explicitly not using the SDK. Please follow the SDK's documentation and the SDK tutorials (e.g. the one you linked in your first post) for how to authenticate with the SDK.