Actually I am using Ubuntu but I face a similar error, saying that cudatoolkit 10.1 is not available in any channel. My pytorch was installed in my conda (base), but I would like to also install pytorch in my newly-created env (dnn).
Collecting package metadata (current_repodata.json): done
Solving environment: failed with initial frozen solve. Retrying with flexible solve.
Collecting package metadata (repodata.json): done
Solving environment: failed with initial frozen solve. Retrying with flexible solve.

PackagesNotFoundError: The following packages are not available from current channels:

  - cudatoolkit=10.1

Current channels:

  - https://conda.anaconda.org/pytorch/linux-64
  - https://conda.anaconda.org/pytorch/noarch
  - https://conda.anaconda.org/conda-forge/linux-64
  - https://conda.anaconda.org/conda-forge/noarch
  - https://conda.anaconda.org/conda-forge/label/dev/linux-64
  - https://conda.anaconda.org/conda-forge/label/dev/noarch

To search for alternate channels that may provide the conda package you're
looking for, navigate to

    https://anaconda.org

and use the search bar at the top of the page.

This env is fresh and clean and I have been looking for a solution for quite a while but with no success.