Maybe this could help someone with these configuration:

• Operative System version: macOS Big Sur 11.2.3

• QGIS: 3.16.5-Hannover

The short answer:

In our computer console:

python_qgis_path -m pip install -U pip

python_qgis_path -m pip install -U scikit-learn

Where python_qgis_path is the python used in QGIS. In my case python_qgis_path is /Applications/QGIS-LTR.app/Contents/MacOS/QGIS/bin

A detailed answer

The python_qgis_path can be deduced from the Python Console from QGIS:

import sys
print(sys.executable)

In my case:

• The print message is /Applications/QGIS-LTR.app/Contents/MacOS/QGIS
• And the python_qgis_path is /Applications/QGIS-LTR.app/Contents/MacOS/bin/python3

When I run the python_qgis_path in my computer console, I get:

In my computer, when I tried python_qgis_path -m pip install -U scikit-learn i got an error with the Numpy dependency. Update the pip version is a way to handle this.

For this, reason I executed:

/Applications/QGIS-LTR.app/Contents/MacOS/QGIS/bin -m pip install -U pip

/Applications/QGIS-LTR.app/Contents/MacOS/QGIS/bin -m pip install -U --ignore-installed scikit-learn

Due my previous attempt with scikit-learn, I had to add the flag --ignore-installed

from dzetsaka.