virtualenv can't find modules - python-3.x

Question

So I had been working with virtualenvs a couple months ago but had to stop for a while and now I don't seem to be able to get it working again.
Here is what I did
D:\CS\Python_Projects\HomeCenter>venv\Scripts\activate
(venv) D:\CS\Python_Projects\HomeCenter>pip freeze
You are using pip version 6.0.8, however version 7.1.2 is available.
You should consider upgrading via the 'pip install --upgrade pip' command.
Flask==0.10.1
Flask-SQLAlchemy==2.0
itsdangerous==0.24
Jinja2==2.8
MarkupSafe==0.23
SQLAlchemy==1.0.8
waitress==0.8.9
Werkzeug==0.10.4
(venv) D:\CS\Python_Projects\HomeCenter>server.py
Traceback (most recent call last):
File "D:\CS\Python_Projects\HomeCenter\server.py", line 3, in <module>
from waitress import serve
ImportError: No module named 'waitress'
(venv) D:\CS\Python_Projects\HomeCenter>
Same happens if I were to run app.py, it won't find flask. I have been using PyCharm and when I check there it tells me the interpreter is pointing towards 3.4.2 virtualenv at D:\CS\Python_Projects\HomeCenter\venv
So why can't it find the modules?
Oh contents of server.py in case it's relevant
from waitress import serve
import os
from app import app
port = int(os.environ.get('PORT', 9999))
serve(app, host='0.0.0.0', port=port)

Answer 1

Hello I am korean student so I can't English not well. Please understand.
In my case, I have a well operated program but suddenly occured "not found" error like you after update mac os to BigSur.
but I finally found solution.
when before update I compiled by "source venv/fileName.py" but after update I compile by "python3 venv/fileName.py" then well operate
You try it too.

Answer 2

A couple of things could have happened here:
Use which python and which pip to check if both are from the virtual env itself.
As dirn suggested in the comments, it might be a PATH mixup. The packages were installed in the virtual environment but the script was executed from outside the environment interpreter. (Use venv\Scripts\python.exe <filename.py> instead of just filename to execute).