Pessoal, alguém pode me ajudar a resolver este problema??
Isto está acontecendo com vários outros pacotes.
PS: Acabei de instalar o R na minha máquina.

> install.packages(mapdata)
Error in install.packages(mapdata) : object 'mapdata' not found
> install.packages("mapdata")
--- Please select a CRAN mirror for use in this session ---
Warning: unable to access index for repository https://cran-r.c3sl.ufpr.br/src/contrib:
  cannot open URL 'https://cran-r.c3sl.ufpr.br/src/contrib/PACKAGES'
Warning: unable to access index for repository https://cran-r.c3sl.ufpr.br/bin/windows/contrib/3.4:
  cannot open URL 'https://cran-r.c3sl.ufpr.br/bin/windows/contrib/3.4/PACKAGES'
Warning message:
package ‘mapdata’ is not available (for R version 3.4.2) 
> library(mapdata)
Error: package ‘maps’ required by ‘mapdata’ could not be found

Desde já, obrigada!!