While running R as the administrator, reinstall ggplot2 like this:
devtools::install_github("tidyverse/ggplot2")
require(ggplot2)
You should be able to see it now with ?geom_sf
For questions related to using devtools, see this SO answer.
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…