I encountered the same issue. As people previously stated, the problem lies in that xlatex isn’t found in your PATH environment variable.
A solution that worked for me was to run the following from the notebook:
!export PATH=/Library/TeX/texbin:$PATH
Or in a command line simply:
export PATH=/Library/TeX/texbin:$PATH
And then run the export to pdf from a command line (within your virtual environment if there is one in place) as follows:
jupyter nbconvert your_notebook.ipynb --to pdf
This should create a pdf of your notebook on the same directory it is running.