I had this problem also, when running "pip install eth-brownie" in the command line, I was given a warning that said
C:\Users\my_username\AppData\Local\Packages\PythonSoftwareFoundation.Python.3.9_qbz5n2kfra8p0\LocalCache\local-packages\Python39\Scripts
is not on the PATH.
You could try running pip uninstall eth-brownie
and re-installing pip install eth-brownie
to see which file you need to add to PATH.
Copy the file path it gives (should look similar to what I posted above)
To add this to path (on windows 10) type advanced system settings into the search bar, click view advanced systems settings, a window that says "system properties" should pop up, click environment variables..., in the next window that pops up (titled environment variables) click on path then click edit, a new window will pop up titled edit environment variable click new then paste the file path into a new line. Click ok and close all of your programs (if you don't close/restart your programs they will not be aware of the change).
When you open your IDE (visual studio code in my case) you should be able to run brownie by typing brownie