Please do not use screenshots here, they are not very useful, paste the actual text output instead.
From what I can manage to see (the image is not very clear) you are being asked to choose a CRAN mirror, try using RStudio's CRAN repository
install.packages('chron', repos='http://cran.rstudio.com/')
If you still keep getting the "Unable to access index for repository" error message, is possible that you are facing some kind of traffic blocking so check if your internet connection is behind a firewall or proxy server, and if it does, then ask your IT department for the proper credentials.