Travis tests for R package fail with little explanation
If you are using the recommended setting for R packages of upgrading warnings to errors, i.e. with this line in .travis.yml:

warnings_are_errors: true


then the place to look is the previous heading in the Travis log, called Checking package.

Example of problem with missing font

In my case, the Travis log under the heading Checking package showed that the
inconsolata font was missing, so the vignettes failed to build:

* checking PDF version of manual ... WARNING
LaTeX errors when creating PDF version.
This typically indicates Rd problems.
LaTeX errors found:
! LaTeX Error: File `inconsolata.sty' not found.
Type X to quit or &lt;RETURN&gt; to proceed,
or enter new name. (Default extension: sty)
! Emergency stop.
&lt;read *&gt; 

l.276 ^^M

!  ==&gt; Fatal error occurred, no output PDF file produced!
* checking PDF version of manual without hyperrefs or index ... ERROR
* DONE
Status: 1 ERROR, 1 WARNING


This caused an error for LaTeX, a warning for R, and an error for Travis.

Solution to problem

In my case, the problem was due to Travis using TeXLive 2019 and the CTAN
mirrors having yet to update to 2019.

The issue appears in the Travis R community forum:


  might be related to the recent switch to TexLive-2019 as the LaTeX package manager seems to fail finding inconsolata in the repository.
  
  ...
  
  It looks to me like the mirror chosen was has not updated to 2019, so I think this will resolve itself naturally when the mirrors catch up. If not I can look into it.


In the meantime, I solved it by forcing a previous version of TeXLive with a downgrade to OSX 10.12 in .travis.yml:

# macOS 10.12 version
osx_image: xcode9.2


See this Travis
page for the
correspondence between XCode and OSX versions and the original thread in DevOps
StackExchange.