Hi @STG8700,
Welcome to the RStudio Community Forum.
The install.Rtools() function mentioned in the error message comes from the installr package. Have you run library(installr)?
Also note that the latest R version, R-4.0.0, requires a new version of Rtools to be installed on your machine. The relevant link if you use Windows is (at your local CRAN mirror): https://cran.ms.unimelb.edu.au/bin/windows/Rtools/
HTH