PDF via LaTeX Download Error

I get an PDF via LaTeX error on some of the Labs when trying to save the lesson.

Does Anybody know how to fix this problem?
It seems to be internal to the course and not a problem with my system since some labs download just fine.

Specific Labs that fail to download (error message follows list) are:
• W2L1 ReLu Activation
• W2L4 Derivatives
• W2L5 Back Propagation
• W2 Assignment – Neural networks for Multiclass Classification

500 : Internal Server Error

The error was:

nbconvert failed: PDF creating failed, captured latex output:
Failed to run “[‘xelatex’, ‘./notebook.tex’, ‘-quiet’]” command:
This is XeTeX, Version 3.14159265-2.6-0.99998 (TeX Live 2017/Debian) (preloaded format=xelatex)
restricted \write18 enabled.
entering extended mode
(./notebook.tex
LaTeX2e <2017-04-15>
Babel <3.18> and hyphenation patterns for 3 language(s) loaded.
(/usr/share/texlive/texmf-dist/tex/latex/base/article.cls
Document Class: article 2014/09/29 v1.4h Standard LaTeX document class
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No file notebook.aux.
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ABD: EveryShipout initializing macros
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geometry driver: auto-detecting
geometry detected driver: xetex
geometry verbose mode - [ preamble ] result:

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Package hyperref Warning: Rerun to get /PageLabels entry.

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! LaTeX Error: \mathbf allowed only in math mode.

See the LaTeX manual or LaTeX Companion for explanation.
Type H for immediate help.

l.1566 \mathbf{v_m^{(k)}}
$ and (\mathbf{v_m^{(i)}}) :
?
! Emergency stop.

l.1566 \mathbf{v_m^{(k)}}
$ and (\mathbf{v_m^{(i)}}) :
Output written on notebook.pdf (20 pages).
Transcript written on notebook.log.

What method are you using to “save the lesson”?
To where are you trying to save it?