R Markdown: openBinaryFile: does not exist (No such file or directory)

I've developed a shiny app that allows user to download a HTML report via R Markdown. I'm trying to include custom css and images into my rmarkdown file. However, I keep getting this error message:

pandoc: Could not fetch (either css or image file)
openBinaryFile: does not exist (No such file or directory)

When I knit the .rmd file on R Studio, it is able to reference the image file or css that I want. However, when I run the Shiny app and download the html file, I get the above error message. I've even tried to put the images and css files in the same working directory as the .rmd file, but to no avail...

output:
  html_document:
   css: pandoc.css

(same error message as above)

Been trying to find a solution for this but can't seem to...can anyone help here?