Rtools is a extra package necessary to compile a new package in R: 

Downloand and intall for windows in: https://cran.r-project.org/bin/windows/Rtools/

and see for Mac: https://clanfear.github.io/CSSS508/docs/compiling.html

See more in: Mastering Software Development in R
Roger D. Peng, Sean Kross, and Brooke Anderson

2020-12-20
https://bookdown.org/rdpeng/RProgDA/before-you-start.html