Hi @Roxana,
This is an error message that has been asked for help many time in this website. Have you consult the solutions provided in older threads ?
This is very often a connectivity issue. Are you in an offline environment ? or do you work behind a proxy (often the case in an enterprise network)? In both cases, you'll have this error message because you can reach the cran url.
If you have a proxy, you need to configure it before trying to "go out" on the web.
Please, read past threads to get hints on what people have done to fix this and let us know here if you need further help.
Thanks !