14.04 - How to fix APT error during the "install the base system" in Ubuntu Server installation?

When installing Ubuntu 14.04 LTS Server, I get this error message:

There are problems and -y was used without --force-yes
WARNING: The following packages cannot be authenticated!

Every time I reach the "install the base system" step at end, and select a kernel to install like linux-generic, it shows that an installation step failed. And in the console, the above error message appears.