I also tried devtools::install_github('rstudio/rmarkdown') but was still getting an error when I added 'fig.align='center' to a ggplot2 plot in my document. It would work as HTML, but not as PDF.
After seeing isomorphismes's post I clicked on the gear symbol next to the knit PDF button, then under the advanced tab I changed the LaTeX Engine to xelatex. After that I no longer received the error message and my PDF document was created without problems.
Thank you.
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