I Have an xbox 360 with Plex and it works fine. I brought an xbox one yesterday signed into xbox live and there is no Plex app in the Xbox app store.
Depending on which region you’re in the app might not be in the store.
I’m in the UK and it shows and I’d assume it also shows in the US region. You might want to try changing your console’s region to one of those, downloading the app and changing back?
Thanks for your Answer, I have tried that and your absolutely correct. The OK App store on the Xbox One Sucks big time in HK. Got it from the UK store.
Thanks Again.