No this option is not available at the moment, but that's how all Google Free Trials are working: You always need to add a payment method, so that the amount can later be charged when the Free Trial is over.
There is no option at the moment to avoid adding a payment method and setup a Stadia account without one. I guess this is because Google standardised for "a limited time" that every new user can start with 1 month Stadia Pro for free. More information you can find here.
They also mentioned it in this article:
- Do I have to sign up for a Stadia Pro subscription when I create a new Stadia account?
For a limited time, when you create a new Stadia account, you'll be eligible for a 1-month trial subscription to Stadia Pro. If you have a Stadia account but not an active Stadia Pro subscription, and you've never subscribed to Stadia Pro, you can still receive the offer when you subscribe. - To sign up for Stadia Pro, go to stadia.google.com/pro. Terms apply.
In the Terms you actually do also find that a payment method is required for sign-up.
I mean as Google states that this offer with the Stadia Pro is a "limited time offer", you could also try to wait and create a Stadia account at a later time.
You could also try to change your payment method either on Google Play or on Google Pay (pay.google.com), but usually Google shows you already when signing-up which payment methods you can use to set up your Stadia account, so I guess you don't have much of a choice there.
