Not much options to do anything wrong, I think.
But maybe not all packages that you need for knitting are installed yet (?)
I think you need at least the package rmarkdown because installing that will also install packages as knitr and other packages. Did you install that?
You can see which packages are installed by typing in the R console:
installed.packages()[,1]
or checking the packages panel.
From the packages panel you can also install rmarkdown and dependencies (make sure that the install dependencies field is flagged).
You did not tell on what type of system your are working and maybe it is not relevant in this case.
Bit if installing missing packages does not work, it could become relevant.
Please inform us by copying the (first few lines of) the output of
sessionInfo()
in this message.