无法打开怎么办啊很急帮帮忙谢谢

Supplementary Text.
Warning: unable to access index for repository https://mirrors.tongji.edu.cn/CRAN/bin/macosx/el-capitan/contrib/3.6:
cannot open URL 'https://mirrors.tongji.edu.cn/CRAN/bin/macosx/el-capitan/contrib/3.6/PACKAGES'