Realizando todos esos pasos ahora tengo este mensaje.
> install.packages("ggplot2")
WARNING: Rtools is required to build R packages but is not currently installed. Please download and install the appropriate version of Rtools before proceeding:
https://cran.rstudio.com/bin/windows/Rtools/
Installing package into ‘C:/Users/activo/Documents/R/win-library/3.5’
(as ‘lib’ is unspecified)