Hey, if you're still having issues with this I figured out one way to work this through. Don't use the sandbox used in the Partner Center.
Instead, use the sandbox XDKS.1 on the X-Box one. This allowed me to sign into my X-Box One through the developer account (the normal X-Box sign in email ie yahoo, hotmail; and not the crazy whatever other one; yeah, you can sign in now as a dev...Horray!) and to begin to work with Visual Studio to work through an initial C# program (it was another avenue I researched that was supposed to be done before being able to get X-Box UWP to work properly because of debugging and releasing remote access of a Visual Studio 2017 program (2015 or 2017 but not 2019) may work through some bugs (by the way, local 64 debug and release had to happen first for me before remote); but I couldn't do it with the initial sandbox I was issued in the center because the same message would come up) So I switched it up!
This XDKS sandbox allowed me to see remote access work from Visual Studio to X-Box while putting in my X-Box IP address in a blank C# program in the debug section (I didn't see it in a blank C++ program fyi).
I got the blank app running on the X-Box One. From there I worked through the kinks. While I happened to initially get Windows version working on GMS 2. Today, I didn't; but the UWP X-Box version is now running (which was my goal for the Creators Program) It's an amazing feeling to see it come up on the X-Box One. I'm sure there's more to it, but I don't care too much right now.
I literally don't want you going through the same stuff for a year to come to where I am at this point (taking months off somewhat discouraged here and there).. and I know looking into contacting Microsoft would charge money for them to answer the question because I saw your article awhile back and almost thought I needed to myself but it's like a contract and money involved and looked into it myself (at least, that was the impression I got and darn near gave up again for another so many months because nobody could give a good enough answer to this question on any forum. So this is one way I found to skin this cat. Being able to sign in makes a difference.)
I dunno if I'll ever get actual use from the sandbox I was given, but the XDKS.1 sandbox I found while searching YouTube videos works for me because I can use it with my X Box account. I just got my game to load up. It's not even close to being ready, but I just wanted walls and colored blocks to pop up. (I got my menu, my walls, but not my blocks/characters and this is where I am at at this point...and that's fine)
There must be a reason to be able to use the issued sandbox, but for now I'm bypassing it for as long as I can because it has been a stressful process to get to this point. So I'm happy to tell you this at 2AM. This is a step in a direction where light can be seen at the end of the tunnel. Good luck with your project BestGame! Have a Happy Holiday!
Old School