I’m having this problem for the second time this year. I’ve tried the hotfix suggested above without success. I bought a realm a few days ago on iOS and it didn’t set up. But my money was taken - no problems there. I can cancel the subscription but the initial amount has gone and I don’t see anyone worrying about that on my behalf. I’ve been playing Minecraft for some years and have had a realm before successfully on the same device, but errors and problems on Minecraft seem like a way of life for players. But if players are purchasing then resolution should be a lot quicker than it appears to be.
I tried accessing my Minecraft account on an iPhone. It recognised that a realm was waiting to be set up, but when I tried I got the same result. I then deleted Minecraft on the iPhone and tried again. Same thing, no solution.
I then cancelled my subscription on Apple and installed Minecraft on an Amazon Fire tablet. I logged in (my purchases are saved to my account, not individual devices).
I purchased a subscription for Realms Plus, same as before. I was very relieved when it worked instantly and I can now use this from my iPad. Some happy young family members and myself of course.
I’ve lost some money on the initial subscription, but I can’t wait around for ever more waiting for a fix.
Good luck everyone!