This is due to your distribution adopting PEP 668 – Marking Python base environments as “externally managed”.

TL;DR: Use a venv:

python3 -m venv .venv
source .venv/bin/activate
python3 -m pip install -r requirements.txt

Your distribution is trying to protect you against mixing apt provided packages and pip provided packages. Mixing two package managers (apt and pip here) is always a bad idea and the source of many issues.

Source:

1. https://stackoverflow.com/questions/75602063/pip-install-r-requirements-txt-is-failing-this-environment-is-externally-manag/75696359#75696359