I have installed Pandoc so that seems not to be the issue
Just to be sure as pandoc-citeproc is another binary and not the same as pandoc. Is it the pandoc version shipped with RStudio ?
on my system, pandoc-citeproc is with RStudio pandoc
list.files(Sys.getenv("RSTUDIO_PANDOC"))
#> [1] "pandoc-citeproc.exe" "Pandoc User's Guide.html"
#> [3] "pandoc.exe"
Or is it a custom install ?