It seems that the package is not installed:
See output of Jupiter lab extension list
(anaconda3) MacBook-Pro-de-Florent:~ fcomte$ jupyter labextension list
[W 2021-06-30 14:50:25.364 LabApp] ‘browser’ has moved from LabApp to ServerApp. Be sure to update your config before our next release.
JupyterLab v3.0.14
/Users/fcomte/opt/anaconda3/share/jupyter/labextensions
@jupyter-widgets/jupyterlab-manager v3.0.0 enabled OK (python, jupyterlab_widgets)