If you're talking about the terminal, as opposed to the console, I don't think it's set up to be used as an R REPL by default (since that's what the Console in RStudio is used for).
So, if I want to execute an R command in the terminal in RStudio, I first have to open R in the terminal (in the gif I show what happens if I send 5 + 5 to the terminal before and after opening R)
Edit Whoops, sorry that one got trimmed before I actually ran 5 + 5