Where are you located? Which country, specifically.
The reason for asking is: that message will occur if you are attempting to download localization-specific software that is inappropriate for the the Mac's location. Macs are configured for sale in broad geographic areas, so if you purchased yours in North America it might need to be reconfigured for use in Europe or South America.
It can also occur with certain DNS settings. There are workarounds for that. Determine your Mac's DNS Setting(s) by opening System Preferences... > Network > select your network interface (Wi-Fi, probably) > Advanced... > DNS.