2025/Python/FlaskIntro 
[09:20]❯ python3 -m venv .venv

2025/Python/FlaskIntro 
[09:20]❯ . .venv/bin/activate.fish 

2025/Python/FlaskIntro [🐍 v3.13.1(.venv)]
[09:21]❯ pip install Flask
Collecting Flask
  Downloading flask-3.1.0-py3-none-any.whl.metadata (2.7 kB)

I tried all the files in venv/bin folder and the above command worked.

Cause of the problem: I was using fish shell.