Hello DivYesh,
The tutorial you follow require you to enable the Cloud Speech API for your project, create a service account, and download a private key as JSON. The service account is most important in this case, as the error seems to point to invalid credentials. How did you follow these steps, exactly? What were the results for each of the steps? Have you followed all these steps without any errors? No problems of any kind?
You can read more detail on the "Creating and Managing Service Accounts" documentation
page.
If you need to send confidential or private information, you may securely do so by using the drop-down list of the "reply" button at the top right of the editing window, and send me a private message.