I have just installed the newest overnight build of Rstudio(0.98.939) Rmarkdown on 14.04 Ubuntu, (please take several bows Rstudio folks - you have transformed the user expe
I had this same symptom on Debian Jessie,
sudo apt-get install texlive-latex-extra
was sufficient for me.
I tried the answers above in my ubuntu and it didn't work, finally I found that yihui developed the tinytex package, the only thing you have to do is run the following code:
install.packages(c('tinytex', 'rmarkdown'))
tinytex::install_tinytex()
and you are set, this works cross platform
Cheers