Hi, and welcome!
Please see the FAQ: What's a reproducible example (`reprex`) and how do I create one? Using a reprex, complete with representative data will attract quicker and more answers. Also, please see the homework policy.
See if this gets you back on track
suppressPackageStartupMessages(library(dplyr))
suppressPackageStartupMessages(library(ggplot2))
iris %>%
group_by(Species) %>%
summarize(Sepal.Length_freq = mean(Sepal.Length)) %>% ggplot(aes(Species,Sepal.Length_freq)) + geom_col()
Created on 2020-03-08 by the reprex package (v0.3.0)