The error "Could not validate subscriptions API permissions" is because Google has not yet validated your service credentials. This can take some time, for some developers over 36 hours. I see that you have already seen our trouble shooting guide here, but did you try the workaround that potentially gets these validated faster? In Google Play Console, open your app's dashboard and visit the 'Monetize' section. Go to Products -> Subscriptions/in-app products, and change the description of any product and save the changes. This should work to enable the new service credentials right away (or very shortly) and you can revert the changes to that product. It's not guaranteed to work, but has worked for others in the past to get things working right away. Otherwise, it usually starts working within 24 hours but possibly more than 36.
 
If after all this it still does not work, I recommend you try to make a test purchase as this could jumpstart the validation as well. We recommend doing closed testing with licensed testers on a physical device: https://www.revenuecat.com/docs/google-play-store#create-a-closed-track-and-add-a-tester-to-it