Setting up Google TTS Model

[chalg]

I have the default Open AI TTS working fine, but wanted to also use Gemini. I'm getting the below:
"RuntimeError: Failed to generate audio: 403 The caller does not have permission"
It seems that setting up Google Cloud Text-to-Speech API is more complex than the YouTube data API, which worked out of the box.
I've got Google TTS attached to the same API as YouTube, is this an issue?
Also not sure if I have to "Activate your full account to get unlimited access to all of Google Cloud—use any remaining credits, then pay only for what you use." - as there is a blue ACTIVATE button at the top-right of the screen.
I see a number of requests against Cloud Text-to-Speech API in the console, with a high error %.