When you go on CRAN page for Rcmdr you'll see that it depends on R > 3.5.0.
This is why, the package is not available to you - you should not find it in the table available.packages()
that filters by default according to the R version. See ?available.packages
about built in filter.
As you are on R version 3.4.4, you'll need to install a former version. Previous one, 2.5.0 should be fine according to the DESCRIPTION file from this version
Try something like
remotes::install_version("Rcmdr", "2.5.0")
and see how it goes.