Look at this picture a second...
Focus on the lower part of the main body tube (receiver)
Look behind the pistol grip.
See the green circle behind that?
Well that's where your shoulder stock should be attached to the main body. Not where you've put it
That's all dude
Here's a real one, showing how it travels along the length of the bottom of the receiver
If you think about it, it can't attach where you have it, as that tube has a moving block inside it. It's meant to be free from obstruction. Which is why that little shape and tube is there to hold the shoulder stock safely