I am having the exact same issue! I have logged in and out of itunes & app store and I have forced close Minecraft. I can no longer purchase mine coins on any of my devices. It's infuriating. Mojang told me to contact Apple. I contacted Apple and after three house of support they said it's Mojang's problem!!!!
Same problem on my Ipads and Iphone X. Nothing wrong with my itune - app store account and nothing wrong with my internet connections. Apple say the problem is Mojang.