Is Git Bash installed on the computer? If you are using Windows is the Git Bash directory in your PATH environment variable? If you recently installed Git on this PC, did you close R Studio and reopen it so it is aware of the system changes?
Also, try opening a new terminal. Click the terminal tab, then click where it says Terminal 1 with a little downward triangle and select New Terminal (alternately you can try Alt+Shift+R).