A few answers (Jill's, Marius's, and Roy's) mention the fact that is necessary to choose the correct Python interpreter to make Pylance function properly. I would like to add the fact that this is still necessary to do when using a Jupyter Notebook with the correct Python kernel already chosen.

It is counterintuitive to choose both Python interpreter and notebook's Python kernel to make things work. It is even more counterintuitive considering the fact that Python interpreter's button (on the left bottom of the screen, on status bar) does not necessarily appear when a Jupyter Notebook is open, but when a Python script is open. For instance, in this screenshot, we see the little line under Scikit-learn's import, indicating a problem with the import (even though the import is successful). However, the correct Python kernel, with Scikit-learn installed, is already chosen. Only opening a Python script we notice that the Python interpreter is the reason of this behavior, because a wrong one is chosen, without Scikit-learn. In some sense, one could think that the reason behind this was a problem with the Python kernel or the Conda environment (it is common to experience this kind of problem when experimenting with Jupyter Notebook and Jupyter Lab). I hope this answer may help those who are searching for solving this problem in the specific context of Jupyter Notebooks inside VS Code. They could ignore the other answers because they could think it is not the case for them.

I was facing same issue. I tried all solutions from stack-overflow but none worked. But after lot of searching and time waste I found my silly mistake. I had created folder named 'flask' and stored my project there. I'm beginner and going through lot of such silly mistakes. Hope it would help, if somebody commits same mistake.

For Linux Mint and for those who have installed flask, but VSCode doesn't find it:

1. check the Flask path: pip show flask (should be smth like Location: /home/<username>/.local/lib/python3.8/site-packages
2. in VSCode click left bottom button and choose the python interpreter, in my case I changed it from python3.9 to python3.8 as we can see it in the flask path.

That's because you have not chosen your path correctly, type:
pipenv --venv
then it will show you where your virtual env is installed. Check where the packages are installed in your env, and then type what comes to you from the shell\scripts or whatever\python, and the packages will work.

I had a similar issue while trying to import flask on vscode. I fixed it by using anaconda. Simply you install the flask module in your created environment example screenshot.

How to create a virtual env in anaconda:
1. On the left sidebar, click on environments.
2. Click create (at the bottom).
3. At the pop-up window, give your vir.env a name and select the language version.
4. Once created, you can start installing different modules in your environment.

I hope that helps!

When I did not install the module "flask" in the Python environment currently used in VSCode:

Please use the command "pip --version" to check the source of the module installation tool "pip", the module is installed at this location:

Then, we can use the command "pip show flask" to check the installation location of the module "flask": (It checks whether the installation location of the module is consistent with the Python environment displayed in the lower left corner of VSCode.)

If the "reportMissingModuleSource" message is still displayed here, please reload VS Code.

(F1, Developer: Reload Window)

Are you using a Virtualenv? If so make sure that VSCode is using the virtualenv as your python interpreter, otherwise it will not be able to pick up the packages that you installed inside this virtualenv.

To do so, click on the Python interpreter in your bottom bar, you should get a list of possible python interpreters including your virtualenv.