How to Fix Jupyter Notebook 'Fatal error in launcher: Unable to create process' on Windows — Python path mismatch (Python39 vs Python313)

Fix Jupyter Notebook 'Fatal error in launcher: Unable to create process' on Windows by correcting Python path: remove Python39 leftovers and reinstall Jupyter.

Launching Jupyter Notebook on Windows and hitting “Fatal error in launcher: Unable to create process” is a classic sign that something on your machine is pointing at the wrong Python. If the error mentions Python 3.9 while you are actually on Python 3.13, Jupyter’s launcher is likely wired to an outdated path that no longer exists.

Reproducing the issue

The failure shows up when starting Jupyter from Command Prompt or PowerShell, for example with either of these commands:

jupyter notebook
py -m jupyter notebook

The error message looks like this, with the obsolete interpreter path highlighted by the Python39 segment:

Fatal error in launcher: Unable to create process using "c:\users\USER\appdata\local\programs\python\python39\python.exe"  "C:\Users\USER\AppData\Local\Programs\Python\Python39\Scripts\jupyter.exe" notebook: The system cannot find the file specified.

What’s actually wrong

The Jupyter launcher script is still pointing to an old Python installation at Python39, which is no longer present. Because the path is stale, Windows cannot create the process, and the launcher fails immediately—even if you successfully reinstalled Python and packages like pandas and matplotlib, and even if Python is on PATH.

Fixing it cleanly

First, remove the outdated Jupyter-related files tied to the old Python 3.9 user location if you don’t need them anymore. Delete everything under:

C:\Users\USER\AppData\Roaming\Python\Python39

Then reinstall Jupyter for the Python you actually use now (Python 3.13). Upgrade pip, then upgrade Jupyter using the Python launcher:

py -m pip install --upgrade pip
py -m pip install --upgrade jupyter

After that, the correct files should reside here:

C:\Users\USER\AppData\Local\Programs\Python\Python313\Scripts\

Make sure your PATH points to the right location so the system resolves the updated Jupyter launcher and interpreter.

Why this matters

Jupyter Notebook relies on a launcher script that targets a specific Python executable. When that target points to a removed or mismatched interpreter, the launcher fails regardless of package reinstalls. Cleaning out the old per-user files and reinstalling Jupyter against your current Python runtime prevents the launcher from calling into a non-existent path and restores a predictable startup experience.

Takeaways

If Jupyter throws a launcher error mentioning a different Python version than the one you use, remove the obsolete user-scoped Python version directory, reinstall Jupyter with your active Python, and confirm PATH references the Python313 Scripts folder. This aligns Jupyter with the correct interpreter and eliminates the “Unable to create process” startup failure.