@Nel Do you have multiple payment profiles associated with your Google account? Or maybe an account that's split between regions for the Play Store and Google Payments?
The Help Center has this to say on that exact error message:
I am getting “The item you were attempting to purchase could not be found.” error when I try to make a purchase in the Stadia app on Android?
This can happen when there is a mismatch in your Google Play country and Stadia country.
To resolve this, please create a new payment profile in Google Payments for the country you use for Google Play. You can do this by adding a valid form of payment. Stadia customer support will then be able to assist you in changing your Stadia country.
Alternatively, you can request a change to your Google Play country to align your Google Play country with your Stadia country.
If that doesn't help then you might want to reach out to Stadia support for assistance:
@Nel great to hear! What was the fix? That might help someone else who runs into the same issue.
@codesplice I needed to change the country and profile under preferences on the playstore and then it worked