各位大佬好，我没法intall package，请问是什么原因，要怎么解决？
我在RStudio输入install.packages("sna")，然后显示是这样的：Warning in install.packages :  unable to access index for repository https://cran.rstudio.com/src/contrib:
  无法打开URL'https://cran.rstudio.com/src/contrib/PACKAGES'
Warning in install.packages :
  unable to access index for repository https://cran.rstudio.com/src/contrib:
  无法打开URL'https://cran.rstudio.com/src/contrib/PACKAGES'
Warning in install.packages :
  package ‘sna’ is not available for this version of R

然后我切换到R4.0.5版本输入install.packages("sna")，显示如下：
Warning: 无法在貯藏處https://cloud.r-project.org/src/contrib中读写索引:
  无法打开URL'https://cloud.r-project.org/src/contrib/PACKAGES'
Warning: 无法在貯藏處https://cloud.r-project.org/bin/windows/contrib/4.0中读写索引:
  无法打开URL'https://cloud.r-project.org/bin/windows/contrib/4.0/PACKAGES'
Warning message:
package ‘sna’ is not available for this version of R

A version of this package for your version of R might be available elsewhere,
see the ideas at
https://cran.r-project.org/doc/manuals/r-patched/R-admin.html#Installing-packages