I've very quickly tested this on a restaurant today. I turned a coffee shop into a restaurant and got my current Sims to buy it just for a short time. I staffed it with 2 out of the 3 staff being non binary. Because the uniforms are assigned automatically unlike the retail lots where you assign by gender manually, I didn't get the error message this time. I didn't edit the outfits because I wanted to test what would happen without interfering with the process. Next time I'll just edit them all to be gender neutral.
I think the non binary chef looks perfectly acceptable in their outfit (attached). However the host's outfit wasn't quite as suitable going by their usual style. So since I couldn't assign a different outfit I exited to manage worlds and found the Sim in other households then changed their clothing preferences to masculine, that automatically changed the staff outfit when I went back to the game. It's not the most perfect solution but I think the results are maybe more appropriate. I'm just going from people in my own circle IRL in deciding what clothing would and wouldn't be appropriate but I know everyone is different and that's kind of the point. I think the problem was more that they had to wear the female uniform and they were put in a skirt. It reminded me of a similar story I was told recently of someone struggling with their identity and being told to put on a dress and deal with it, so I think that's why I'm invested in this process because of personal stories like that.
I'm going to design gender neutral uniforms anyway and where I get the error message they can just be without uniforms.
Edit: I've now tested the Vet's too. I edited the chosen member of staff in Other Households to make them non binary. It worked just the same as the restaurant with uniform automatically assigned based on their clothing preferences from CAS. I didn't get an error message.
