Hi @Bami,
Welcome to the RStudio Community Forum.
This error means that R cannot find the devtools package.
Did you run both of these lines of code?
install.packages("devtools") # Download package from CRAN and install.
library(devtools) # Load package ready for use in current R session
Did they both complete without errors? If not, you will need to post the output from the console for us to diagnose the problem further.
HTH