Hi Community Members,
Since the release of Dash 2.1.0, you might have seen an error thrown when trying to use Jupyter-Dash 0.4.0 or older versions.
Read-only: can only be set in the Dash constructor or during init_app(), requests_pathname_prefix
Thank you to our community member @Oladayo and others for reporting this error.
With the newest Jupyter-Dash version 0.4.1, this error has now been fixed.
Official Changelog 
Jupyter-Dash 0.4.1