I have also completely uninstalled Minecraft Windows 10 edition and installed it again. I also went to Settings > Apps and Features > Minecraft > Advanced Options and hit Reset. Still nothing.
Incidentally, I can create a private game with no issues. Realms is the one that is erroring out. This applies to any world pack I have.
I have also completely uninstalled Minecraft Windows 10 edition and installed it again. I also went to Settings > Apps and Features > Minecraft > Advanced Options and hit Reset. Still nothing.