Changing the default RMarkdown font - r

Question

I am struggling with changing the font of my pdf files. I don't have Helvetica on my computer so I am unable to compile .pdf (html works perfectly). I believe my Rmarkdown coding is not working but it is identical to what is suggested here. LaTeX is installed and properly working, I can compile using XeLaTeX from Lyx.
---
title: "5_jan"
author: "Matias Andina"
date: "January 5, 2017"
output:
pdf_document:
latex_engine: xelatex
sansfont: Calibri Light
---
As far as I understand, Calibri font is loaded
fonts()[grepl('Calibri',fonts())]
[1] "Calibri" "Calibri Light"
I keep geting the cannot load afm file 'Helvetica.afm' error. This is my session
sessionInfo()
R version 3.3.0 (2016-05-03)
Platform: x86_64-w64-mingw32/x64 (64-bit)
Running under: Windows >= 8 x64 (build 9200)
locale:
[1] LC_COLLATE=English_United States.1252
[2] LC_CTYPE=English_United States.1252
[3] LC_MONETARY=English_United States.1252
[4] LC_NUMERIC=C
[5] LC_TIME=English_United States.1252
attached base packages:
[1] stats graphics grDevices utils datasets methods
[7] base
other attached packages:
[1] extrafont_0.17 knitr_1.13 dplyr_0.4.3
loaded via a namespace (and not attached):
[1] Rcpp_0.12.7 digest_0.6.9 assertthat_0.1 R6_2.1.2
[5] Rttf2pt1_1.3.4 DBI_0.4-1 magrittr_1.5 highr_0.6
[9] lazyeval_0.2.0 extrafontdb_1.0 rmarkdown_0.9.6 tools_3.3.0
[13] yaml_2.1.13 parallel_3.3.0 htmltools_0.3.5
UPDATE
The error is with R code chunks. If I knit everything but the R chunk it works
---
title: "5_jan_hplc"
author: "Matias Andina"
date: "January 5, 2017"
output:
pdf_document:
latex_engine: xelatex
mainfont: Calibri Light
---
this is my test
I can write anything here and it will render **perfectly**
```{r}
plot(mtcars)
```
processing file: test.Rmd
Quitting from lines 16-17 (test.Rmd)
Error in grDevices::pdf(NULL, width, height, ...) :
failed to initialise default PDF font
Calls: <Anonymous> ... chunk_device -> dev_new -> do.call -> <Anonymous> -> <Anonymous>
In addition: Warning messages:
1: In grDevices::pdf(NULL, width, height, ...) :
afm file 'C:\PROGRA~1\R\R-33~1.0/library/grDevices/afm/Helvetica.afm' could not be opened
2: In grDevices::pdf(NULL, width, height, ...) :
cannot load afm file 'Helvetica.afm'
Execution halted