Despite being the 9th most massive body orbiting the Sun (beating out Pluto and all the other dwarf planets), the same size as Pluto, and more reflective, Eris is currently just past aphelion, its most distant from the Sun, at 95 astronomical units (95 times as far as the Earth is from the Sun) or nearly triple the distance of Neptune or Pluto from the Sun. As such, it is extremely dim, and at magnitude 18.5, even a 25” telescope is probably insufficient to see it. A 30” or larger telescope is likely necessary for any hope of picking it out even under dark skies. Only a handful of people have ever seen Eris, though it was observed visually with an 82” telescope just months after its discovery in 2006.
Things won’t always be so bad, though. By the end of this century, Eris will peak in brightness above magnitude 18, and thus be within range of a 20” or larger telescope if dark skies still exist. And in the mid-2200s, Eris will be at perihelion, slightly closer to the Sun than Pluto is currently and peaking at magnitude 14.5, within range of a 10” or so instrument.