The access to the anaconda "main" channel has been blocked due to licensing changes:
https://www.anaconda.com/blog/update-on-anacondas-terms-of-service-for-academia-and-research
You need to completely remove the main channel from your personal config, i.e., from ~/.condarc.
In the channels section, the main channel is usually called defaults or given as two URLs:
Instead, you need to insert conda-forge into the channel list if it is not already on the list.
You can use the following command to check if your config is correct:
conda config --show-sources
The output should not contain any variant of the main channel.
If you do not have a ~/.condarc, you need to create the file with the following contents:
report_errors: false
channels:
- conda-forge
For new installations, it is easiest to use miniforge, which is miniconda where defaults is replaced by conda-forge out of the box: