The ports/espressif build setup is a rather involved process. Please read the README.md in that directory. It has instructions that also refer to further instructions in the ESP-IDF documentation.
On MacOS, you will need to install cmake and ninja:
brew install cmake brew install ninja
On Linux you probably need to install ninja-build and cmake.
sudo apt install ninja-build cmake
The ESP-IDF expects there to be a python command which runs python3. On Ubuntu, there is no plain python by default, so install this simple package which links python to python3.
sudo apt install python-is-python3
Once you have the prerequisites installed, change to the ports/espressif directory, and run the install.sh script. You only need to do this once.
cd circuitpython/ports/espressif esp-idf/install.sh
After this, in each fresh terminal window in which you are doing builds, you need to use esp-idf/export.sh in order to set up the correct PATH and other environment variables.
# Do this in each new terminal. cd circuitpython/ports/espressif source esp-idf/export.sh
export.sh will suggest that you use idf.py to do a build after you run it. But we don't use idf.py. Instead, use make, for example:
make BOARD=adafruit_magtag_2.9_grayscale
