Exactly what code did you run when you installed it?
I most often see that error when I misspelled the package name, for example on my machine,
install.packages("misspelled-pkg-name")
#> Warning: package 'misspelled-pkg-name' is not available (for R version 3.5.0)
Created on 2019-10-03 by the reprex package (v0.3.0)