TL;DR:
There are two answers to this, really:
- In Universe, the Empire is prejudiced against non-humans, it is entirely made up of humans exclusively. This prison is an example of the segregation that exists in this universe. There may or may not be an alternative facility that caters for the non-humans.*1
- Out of universe, the writer chose to concentrate on the human story rather than get bogged down in alien side issues such as anatomy and accommodating that in the prison, for example.
*1 (Given the idea of 'Human High Culture' that is used to explain humans everywhere, and their hierarchy, it seems unlikely. The non-human prisoners were in all likelihood sent off to far worse facilities that catered even-less to the particularities of the various species)
Gilroy, the writer has been asked about this a few times apparently:
Lack Of Star Wars Aliens In Andor Explained By Showrunner
There's some, and we'll probably have more. It's a very strong flavor when it comes in. It's not just a visual flavor, it's a very strong character flavor. You have to deal with it politically. And in some places people were saying, “Oh, why is Narkina 5 all humans?" Well, I don't know how you would work out the bathroom on the floor with eight different varieties of genitals, or whatever. I mean, it has to be that way, a system like that, maybe there's Narkina 2 where there's different things. We're probably a little bit shy about it because it's such a behavioral show and most of our principles are in this particular world. And certainly, the Empire doesn't have a surplus of aliens on their side. We will have more, and we'll have them in the appropriate places. And we try to pick our shots and make them cool, I guess. I don't know.
https://screenrant.com/andor-star-wars-aliens-explained-tony-gilroy/
Aside from the above, this article refers to the Empires prejudice against non-human species in general anyway:
https://screenrant.com/why-there-are-no-aliens-at-andors-labor-camp/
Curiously, the aliens of the Star Wars universe have absolutely no presence in Narkina 5's Imperial labor camp in Andor episode 8. Why?
As for where they all are, it seems the answer lies primarily with the Empire’s approach to its rule. It would appear that the Empire preferred to maintain some degree of separation between the different species in the Star Wars universe. How the Empire treated people was defined partly by whether or not they were human, which makes sense of the aliens’ absence from Narkina 5.
Among the many flaws shared by the Galactic Empire in Star Wars is its anti-alien bias. It’s no secret that most Imperial officers are humans, and there’s a good reason for that. The Empire has a xenophobic attitude and has long harbored distrust toward the numerous alien species that occupy its sectors.
Perhaps unsurprisingly, this outlook is notably reflected by its treatment of its prisoners. While Andor episode 8 did highlight how cruel and callous the Empire can be to its human subjects, other Star Wars projects have shown they can be even more malicious to their alien prisoners.
More from Gilroy:
There’s already so much politics in the show to begin with, and we’re trying to tell an adventure story really. So adding strong alien characters means that all of a sudden, there’s a whole bunch of new issues that we have to deal with that I don’t really understand that well or I just couldn’t think of a way to bake them into what we’re doing. You’ll see more as we go along, but it’s a legit question and one we’ll be answering as we go along. There is a more human-centric side of the story and the politics of it. There’s certainly no aliens working for the Empire, so that kind of tips it one way, automatically.
https://winteriscoming.net/2022/11/15/star-wars-andor-creator-explains-why-there-arent-many-aliens/