can you click the link http://cran.rstudio.com/src/contrib/PACKAGES in your browser and get served the contents like any other website ?
if not, then your internet access is generally restricted ... are you behind a corporate or educational firewall etc?
if you can browser over to there, then in principle R should be able to also, so hope is alive; and you can search for things to try.
start here for example:
install.packages unable to access index for repository: Try disabling “secure download method for HTTP” - General - RStudio Community