The causes could be:

1. The name of the file - fastapi.py, if you name it in this could way you would get import errors. (Avoid filenames similar to package names)
2. Installing on other environments. Most of the time it happens when you use both Conda and PiP on you system.
3. If you run the code from VS Code, check if the same interpreter is selected.

Verify the installation:

$ pip list | grep fastapi

Suggestions:

OPT Virtual Environments to avoid these troubles.

Thank you!

If you installed FastAPI, this is probably because you're not using the right Python environment in VS Code. Check this tutorial by Microsoft, Section "Locally in VS Code", to set up FastAPI in VS Code. Make sure to follow all steps exactly, especially those related to the environment!

Can you give us your directory structure and the directory you are in when you run your uvicorn command?

You should be in the same directory as your main file to run uvicorn main:app so it will go to the main.py file and find the app object

TL;DR

Add the directory name in front of your filename

uvicorn src.main:app 

or cd into that directory

cd src
uvicorn main:app 

Long Answer

It happens because you are not in the same folder with your FastAPI app instance more specifically:

Let's say i have an app-tree like this;

my_fastapi_app/
├── app.yaml
├── docker-compose.yml
├── src
│   └── main.py
└── tests
    ├── test_xx.py
    └── test_yy.py

$ pwd         # Present Working Directory
/home/yagiz/Desktop/my_fastapi_app

I'm not inside the same folder with my app instance, so if I try to run my app with uvicorn I'll get an error like yours

$ uvicorn main:app --reload
INFO:     Uvicorn running on http://127.0.0.1:8000 (Press CTRL+C to quit)
INFO:     Started reloader process [40645] using statreload
ERROR:    Error loading ASGI app. Could not import module "main".

The answer is so simple, add the folder name in front of your filename

uvicorn src.main:app --reload

or you can change your working directory

cd src 

Now i'm inside of the folder with my app instance

src
└── main.py

Run your uvicorn again

$ uvicorn main:app --reload
INFO:     Uvicorn running on http://127.0.0.1:8000 (Press CTRL+C to quit)
INFO:     Started reloader process [40726] using statreload
INFO:     Started server process [40728]
INFO:     Waiting for application startup.
INFO:     Application startup complete.

As mentionned by M.O in the comments, FastApi is running on 'http://localhost:8000' meanwhile React on 'http://localhost:3000' this created an issue where the browser will block requests from the React app to the FastAPI API by default due to CORS. Therefore, i needed to enable CORS on the FastAPI API and add http://localhost:3000 to the list of allowed origins like this :

from fastapi.middleware.cors import CORSMiddleware
app.add_middleware(
    CORSMiddleware,
    allow_origins=["http://localhost:3000"], # here
    allow_credentials=True,
    allow_methods=["*"],
    allow_headers=["*"],
)

I hope this will help other beginners and thanks for the help everyone!

1. Open the Command Palette (Ctrl+Shift+P), then select the Python: Select Interpreter. From the list, select the virtual environment in your project folder that starts with .env.

2. Run Terminal: Create New Integrated Terminal (Ctrl+Shift+` or from the Command Palette), which creates a terminal and automatically activates the virtual environment by running its activation script.

3. Install sqlalchemy and mongoengine with command pip install. Once installing them successfully, there will intellisense when you import them and no warnings shown.

Besides, the folder .vscode is to store Workspace settings as well as debugging and task configurations.