I encountered this problems after I insall R and RStudio too.Both of them apear to have problems same as yours.
I can't find the .Renviron in /etc/R.Then I search the Internet and find out I should use this file to set envirenments for my R. So all I need to do is new a ".Renviron" file and write it correctly.
Fortunatelly I find a file named "Renviron.site" in /etc/R and inside it I find a few lines of comments :
# ## Example .Renviron on Windows
# R_LIBS=C:/R/library
# MY_TCLTK="c:/Program Files/Tcl/bin"……
I suppose this would be useful to write my .Renviron file.
So I write these in terminal:
sudo gedit /etc/R/.Renviron
The .Renviron file is automatically created and open by gedit.Later on I copy
R_LIBS=~/R/library
PAGER=/usr/local/bin/less
R_DEFAULT_PACKAGES='utils,grDevices,graphics,stats'
into .Renviron file and save it.And the RStudio just work normally!