I have run into the same error, this post wasn’t very clear so I wanted to add in more detail how I fixed the problem. To my understanding, in order to install openmc using conda, you need to explicitly tell conda that you are emulating x86_64 architecture because the mamba version of openmc is not yet compatible with Apple arm64 chips. I had created such a conda environment in my post here. The drawbacks to installing in this way are that, on apple chips, you tend to get crazy segmentation fault errors when doing things like plotting geometries, and all of those errors told me to install from source, which I did. Once you install from source, your openmc executable will be compatible with arm64 and you then need to create a REGULAR conda environment before pip installing. Then it all seemed to work out. In conclusion, installing from source eliminates a ton of headache down the road.