Hey @Bizmaster, did you buy the founders or premiere edition? Google announced yesterday:
"Here’s the latest update: If you ordered and paid for Founder’s Edition, you should now have your Stadia access code. Pre-orders and access codes for Premiere Edition will start shipping early next week. Thanks for sticking with us!"
Therefore if you are a founder, I would recommend to contact Stadia support if you still haven't got your code, but your package has arrived.