I‘m not sure completely but i suppose this payment method only is available for subscribing from within the Android app. Have you done this way?

Yes, thats within the app

Ok, but if i‘m right google play in app purchases automatically have to be paid with the payment method linked to play store. 
Isn‘t it this way for you? If not, what kind of payment method is set there for you and how did you choose it? 
Some more detailed information would be helpful.