sql 2008 x64, want to install SSRS, "Native" mode option not available.-Collection of common programming errors

We have a dev sql 2008 x64 server (Windows 2008 x64)  and  I want to add the SSRS feature to it. When I added SSRS, the install came to the screen stating my install options. The first option, which I wanted to use, “Install the native mode configuration” was grayed out and unavailable. The only available option was “Install but do not configure the Report Server”, which I did not want to use.  (I have never used it before and am not sure if I will configure it correctly or fully.)

Why isn’t “native” mode selectable?

TIA,

Barkingdog