Hello, i have recently tried to buy pro using my play store giftcard that i got. there is £10 on it, so i definitely have enough. i am using a Samsung galaxy s5 mini, no vpn, and i am 100% sure that my IF is fully updated.
It looks like you are using an older version of Infinite Flight. In-app purchases are no longer supported on those older versions. You will need to buy a device that supports the Global update (and later) in order to buy a subscription.