I am getting the same error for all R Studio installs on my Mojave 10.14.5 Mac as follows:
install.packages("http://tidyverse.tidyverse.org")
Warning in install.packages :
package ‘http://tidyverse.tidyverse.org’ is not available (for R version 3.6.0)
I'm running R 3.6.0 [R.app GUI 1.70 (7657) x86_64-apple-darwin15.6.0] and R Studio 1.2.335.
I've downloaded xcode and opened it to confirm that command line was included. I restarted everything, but am still getting the error. I can install from the binary OS X code, but when there are a lot of dependencies, that's prohibitively time-consuming.
Grateful for any suggestions!