From what I can see, the Windows App Store version of Plex has been discontinued when Plex introduced the new “Plex for Windows” app:
From https://www.plex.tv/blog/desktop-af/ (Aug 2019)
It’s almost midnight, a few hours before this post goes live, but we already hear you asking: What does this mean for the Windows Store app? Whither Plex Media Player? Whoa, whoa, slow down. Let’s take this one at a time. Also no one says “whither” anymore (even though it’s a super useful adverb… just sayin’).
First, the Windows Store app. It’s been a little broken for a while now. Back when we first started developing Windows apps, it made a ton of sense to embrace the ecosystem, but over time ( Windows Phone ) it made less and less sense to keep investing dedicated development resources to the Metro [sic] platform. We’d much rather invest the effort on a cross-platform app with a best-in-class player engine under the hood, so as of today we’re taking our Windows app out of the store. The new desktop app which replaces it has a much more capable player stack, support for all modern features, and finally a proper replacement for Sync.
What surface are you thinking of buying? If its the pro X then plex won’t work and if its the Go you’d need to take it out of S mode to install plex. I have a Pro 7 and while not ideal plex works including downloads