This, or better yet, remove the bolt from the carrier and see if just the sides of they key are binding up against the sides of the  receiver U channel as it hand fitted into the upper.  If yes, remove charging handle and try again.

Hence could be key was not machined enough on the side walls for the U channel of the upper receiver to prevent binding, the upper receiver was not vise blocked correctly so the side of it where not squished inwards when the barrel nut was being tightned, or charging handle is out of spec and what the Key itself if binding against.

If carrier is fine, then install bolt again, use a sharpie to mark the sides of the cam, and see if the binding is there against the U slot isntead (both with and without charging handle in play).