The error message indicates that the server from which RStudio is configured to install and update packages is not responding or cannot be reached. If the server is not responding, it can be expected to be restored to service within a short period. If the server can't be reached through the Internet from the location of your machine, that may be due to temporary network congestion, an intermediate router failure or the configuration restrictions on access to the Internet imposed by services on which your machine depends.
If the problem does not resolve, it is possible to select a different repository. However, the ease and reversibility depends on user experience.