I have only read online several theories that it was created specifically for the show with Unreal Engine or something similar. One theory posits Myst as the model but I don't think that quite fits.
I gave it a search and I'd say, this is a fake game created for the show.
The series is set in the late 80s, early 90s. Specifically in the 4th season, the browser Mosaic is released, which allows us to pinpoint the date of this screenshot to late 1992, early 1993. The graphics don't quite match, they are a few years too advanced. The graphics style reminds me of Magic Carpet (1994), full 3d worlds like this weren't possible in 1992/1993, they first appeared in 1994/1995. Yes, Myst was released in late 1993, but keep in mind, that Myst was a series of mostly static images, while this screenshot looks far more dynamic.
Especially the lighting effects are far beyond what was doable in 1992/1993, they look more like very late 90s, which is an indicator to me, that a modern 3D engine was used.
Any search for this game as well as a reverse image search of your screenshot only relate to posts and articles directly about the show.
In the series full credits I cannot find any mention of design & programming of this game, nor any reference to a real game. This is an indicator, that it was a visual effect, not a real game.
So, is it modelled after a particular game? It certainly does take inspiration from actual games. The graphics style is reminiscent of early 3D engine games. Magic Carpet as mentioned earlier, but also the early FPS. But as the game is third person, it also evokes a vibe of the classic point & click adventures, which was also were very prevalent in this era, but mostly relied on drawn graphics instead of a 3D engine. It also depends, what the game is about, some sites call it an RPG, but others predominantly mention puzzles.
So, in the end I'd say it's a combination of Mysts or Zorks obscure, cerebral puzzles, early 3D graphics from 1994 and a palystyle akin to early/mid 90s point & click adventures.