It sounds like your Rmd file is trying to install a package.
To install a package within a knitted document, you need to state which CRAN mirror repository you want to use.
install.packages("tidyverse, repos = "http://cran.us.r-project.org")
That being said, it is generally not a good idea to install packages in a Rmd document which you are going to knit multiple times.
Ideally you would install the packages needed once, then use
library(package_name) within the Rmd to load each package needed.
One lasting way to set your default CRAN mirror is to create a
with the line
options(repos=structure(c(CRAN="YOUR FAVORITE MIRROR")))
Which will load every time you start R.