Weapons are not attaching to the bone socket.

(English is not my first language, ignore if I sound bad) I have been following a tutorial, and I am trying to learn to implement the game mechanics of PUBG PC. Im stuck here at a point where i am trying to switch between third person mesh to first person arm mesh, but its not working. Weapons do get spwan but do not attach to the socket. However when i tried to test out with the test node using 3 and 4 key, they work perfectly fine. Can you guys go through this video and help me out, Im loosing my motivation at this point. I will be greatful to you all.

I am attaching the link to the video, because it was hard to explain using screenshots.