First, make sure you have Xcode and CommandLineTools installed and updated to latest version (in AppStore). If you have not already installed CommadLineTools, go to a terminal window and run:

  xcode-select --install

First download the latest version of Miniconda3 and run it to install.

  curl -o Miniconda3-latest-MacOSX-x86_64.sh https://repo.anaconda.com/miniconda/Miniconda3-latest-MacOSX-x86_64.sh
  sh Miniconda3-latest-MacOSX-x86_64.sh

Follow the instructions on screen, scrolling down, pressing ENTER and replying yes when necessary. Install it in the default directory. Restart your terminal window to apply modifications. After restarting, you can type the command below to install Mamba:

  conda init
  conda install -n base -c conda-forge mamba