@MojangSupport My friends and I have been trying to buy a Realm for a while now and none of us seem to be able to add payment methods. We have followed all steps in the Realm Payment FAQ, and yet nothing works.
We apologize for the inconvenience, but this is currently a known issue. The team is currently working on a fix that should fix the issue as soon as possible. Thank you for your patience while we work on the solution!