Whats Going On?

I have a late 2011 iMac desktop but when I try to upgrade Safari I get a strange message. It says INSTALL_ERROR_DISTRIBUTION_SIGNED_BY_APPLE. Thats it! There is no explanation about what this means or why I receive this message. Its frustrating. Im not sure what to do, but maybe customer support can help me out.

Tzarita88 about OS X El Capitan