I'm not sure why you would want to plug the output of your scarlett to your windows line-in but a focusrite Scarlett 8i6 is an audio interface that means it can replace your windows built-in sound card altogether.
Hopefully you installed the drivers needed. Your signal chain should be Mic/Instrument --XLR/PL--> Scarlett 8i6 --USB--> Computer
Then you should choose/set your recording/playback devices on your computer to Scarlett device (right click the volume icon on the lower right and select recording/playback devices)