Error "Your default credentials were not found" only with Text-to-speech API

[daext]

When I authenticate with an API Key or with OAuth the site in development generates the message below. I tried three API and this happens only with the Text-to-speech API. With the Google Sheets API and the Google Books APIs this doesn't happen.

Fatal error: Uncaught DomainException: Your default credentials were not found. To set up Application Default Credentials, see https://cloud.google.com/docs/authentication/external/set-up-adc

I can solve in the following two alternative methods:

• By creating a environment variable with a reference to the key. But this is now what I want, because this method is not always supported in the production environment.
• By manually performing the authentication using PHP curl (without using the Google APIs Client Library for PHP)

Why Google Cloud is asking me to configure ADC if I have already provided this information during the authentication with PHP?

[daext]

Posted in the wrong library. Please close it. Thank you.