500 Internal Server Error while opening jupyter notebook or making a new notebook file. 

for finding out what's causing the error , enter the command jupyter notebook in the terminal

Most of us launch anaconda-navigator to use jupyter notebook, if we do that we wont get the token (shown below) of the notebook session.

if you find something like this in the lof of  jupyter-notebook command in terminal. there is an issue with nbconvert

PermissionError: [Errno 13] Permission denied: ‘/usr/local/share/jupyter/nbconvert/templates/conf.json’
[E 13:17:24.525 NotebookApp] {

you might have to down grade it to nbconvert 5.6.1, use pip install nbconvert==5.6.1

#YOU SHOULD RUN IT TWO TIMES.