Go into RStudio and find the system environment variable for RSTUDIO_PANDOC
Sys.getenv("RSTUDIO_PANDOC")
Then put that in your R script prior to calling the render command.
Sys.setenv(RSTUDIO_PANDOC="--- insert directory here ---")
This worked for me after I’d been struggling to find how rmarkdown finds pandoc. I had to check github to look at the source.