The Google Cloud credentials are necessary to access Google Text To Speech service (speech synthesis)

1) log in to Google cloud 
  https:/cloud.google.com
  - you may need a credit card to create the account

2) create your service account
  - [left frame] IAM & Admins / Service Accounts / +Create service account

3) generate the API key
  - in the service account list, click to Actions (the last column) / Create key / JSON / Save
  - name the file google-credentials.json

