You need to provide what ip address you are trying to reach the server
from for anyone to help on this.
There could be two reasons for this:
The Scientific Linux distribution is sponsored by a United States
Department of Energy National Lab, Fermi Laboratory. That does mean
that it has to abide by certain restrictions on what countries are
allowed to connect to the server (countries like North Korea and some
others).
Various foreign governments block access to United States government
servers for various reasons at various times. This can cause problems
for other people if your access 'transits' that countries routers.
Not much can be done about this other than trying to find a mirror
that is allowed access in the country and then pointing your
yum.repos.d/*.repo files to that mirror.