ggplot2 is the name of the package, not of the function, also you need to load the library on each new R session to be able to use it, see this example.
# Load the library
library(ggplot2)
ggplot(data = iris, aes(x = Species, y = Sepal.Width)) +
geom_col()

Created on 2019-04-07 by the reprex package (v0.2.1.9000)