There is a way to take screenshots of Roku apps using your computer, but only by sideloading the app you want to shoot.
I’ve never looked into sideloading before, and what little I’ve read, most app developers provide a .zip file for the purpose of sideloading. Is there one for Plex?
My intention was to take screenshots which, according to everything I can find, can only be done with a sideloaded app. But, you said Plex doesn’t supply a zip file, so that answers my question.