The steps to get it installed are not the regular package installation process.
Here's the instructions to install RTools.
https://cran.r-project.org/bin/windows/Rtools/
Why do I need RTools? - For some background, R and R-packages are intended to work on whatever OS you are using. While many packages from CRAN can be installed on your machine via pre-complied binaries, many require additional steps before they can be installed.
Some packages need to have their source code checked, compiled, and have any number of additional operations applied before it is able to be installed and run locally. RTools collects the tools your Windows machine will need to complete that process into one easy installer. (Borrowing heavily from this article)