I'm trying to install Bioconductor on my Mac (10.12 Sierra).  I'm following the instructions here.  I installed R successfully (version 3.4.3) and then typed...

if (!requireNamespace("BiocManager"))
    install.packages("BiocManager")
BiocManager::install()

When I type this command I get...

Selection: 60

Warning message:

package ‘BiocManager’ is not available (for R version 3.4.3)

> if (!requireNamespace("BiocManager"))

•     install.packages("BiocManager")

Loading required namespace: BiocManager

Failed with error:  ‘there is no package called ‘BiocManager’’

Warning message:

package ‘BiocManager’ is not available (for R version 3.4.3)

> BiocManager::install()

Is Bioconductor really not available for version 3.4.3?

Your version of R is too old for BiocManager; you'll want to use

source("https://bioconductor.org/biocLite.R")
biocLite("YourPackage")