I struggled with getting Realms going on my Xbox One for days. However, I was able to overcome the issues I describe below, so read on and if this sounds familiar to you, this workaround may work for you. (skip to the bolded section if you don't want to read the entire post)
First, it would tell me "Your purchase completed successfully, but we could not create your Realm at this time." (this was while trying to set it up via the XB1) Eventually, it would completely reset and act like I hadn't done anything to set up a realm, and my free trial was back in the menu and available. After some time, I decided to hop on my Windows 10 version, and of course, the realm set up just fine. (It's unfortunate that I needed to do this on a different platform, as I realize this is not an option for most people, and I do not have a workaround for you that doesn't involve having to spend extra $ if you are experiencing trouble even setting up a Realm via XB1)
Playing via Windows 10 was something I wanted anyhow so that I could free up my Xbox One for my wife and daughter to play with me from there but on a persistent world (or Realm). However, we ran in to more problems. My Xbox profile could see and connect to the Realm every single time, but both my wife and daughter's Xbox profiles could not even connect to Realms. "We could not connect to Realms at this time. We will try again later."
For days this persisted so I proceeded to troubleshoot. I had determined that all of our profiles are Gold and that wasn't the problem. (also to note, after fixing the issue, I determined neither of them needed Gold to connect to my Realm anyway, by making a new account and inviting it, with it being able to successfully connect). So, after staying up to 5am one morning, I had somehow gotten my wife's Xbox profile to see the Realm and connect to it. I rejoiced and went to bed. However, the next day it was back to not being able to connect to Realms. At first I thought maybe they are just having massive issues with Realms via XB1 and I'd just have to wait for a fix. But this was unacceptable. In a fit of rage, I Hard Reset the XB1 and left the room.
My wife turned it back on and sure enough it was working again. (to which she laughed so hard at me about /sigh)
But that was it. Every time we want the XB1 to connect to the Realm, we have to Hard Reset and it will connect on next startup. However, if you AFK log out, save & exit, lose internet or basically lose connection to the Realm in any way, it will go back to not being able to connect at all. Thus we Hard Reset to get it going again.
EDIT: If you are split-screening on the XB1 while playing on a Realm, and the 2nd, 3rd or 4th player Save & Exits or afk timeout, it will d/c ALL and go back to not being able to connect to Realms, requiring a Hard Reset to get it working again.
Sorry that was long-winded but if this helps even 1 person out there, I think this struggle was worth it.