@inky1003 I think you have the wrong perception about RetroPie. RetroPie is not an OS in itself, it sits on top of a full OS, you can install it on an existing Raspbian, or start with the RetroPie image and add additional software later (quoted from the front page). You can use the full Raspbian (desktop) image and install RetroPie on it, or vice-versa - start with the RetroPie image (recommended) and then add the desktop (https://retropie.org.uk/docs/FAQ/#where-did-the-desktop-go).
If you have installed RetroPie on your existing Raspbian image, you can just start the setup script and do the configurations described in the docs - it's not something included just in the RetroPie image.
Btw, 3.6 is such an old release, no wonder the setup script cannot determine the platform you're running (probably due to the newer kernel included)
