RMarkdown Pandoc Error when my pandoc version is 2.3.1

Question

I'm trying to knitr a Rmd file on Rserver via command line by running

Rscript -e "rmarkdown::render('testrmd.Rmd')"

But it ran into an error saying

"Error: pandoc version 1.12.3 or higher is required and was not found (see the help page ?rmarkdown::pandoc_available).

Execution halted"

However, I checked and found out that I actually have pandoc version 2.3.1, way higher than the required 1.12.3.