Nowadays, I have all my development environment under the Windows Ubuntu subsystem. Also known as WSL, the subsystem was the best addition for Windows if you are a developer.
For those installed Jupyterlab under WSL, the following warning may be familiar. It is annoying, but very easy to solve.
The error seems come from WSL searching for a browser to open the Jupyter UI. Although is not a critical error and will not prevent you from use Jupyter, it deserves to be solved. The warning will disappear as also as a browser will spawn.
First, generate the Jupyter config file if you have not it already.
jupyter notebook --generate-config
Open the config file (usually in ~/.jupyter). Search for the following line and change it from True to False.
c.NotebookApp.use_redirect_file = False
I am using WSL2 (Ubuntu 20.04) and now the error is gone and the browser opens as also as Jupyter starts without warning.