Okay, so this indicates you either don't have pdflatex installed, or it can't be found. If you've never installed a TeX distribution, then the former is probably true.
My recommendation is to use the {tinytex} package and use it to install a lightweight but fully-functional TeX distribution.
You can run this code in R:
tinytex::install_tinytex()
This may take several minutes to install. Once complete, reset your R session and try running Sys.which('pdflatex'). This time it should show you the path to pdflatex which means you should be good to go.
Try rendering the Rmd to PDF and it should work.