I am modelling my first blender character ever. I modeled one side, then symmetrized it and merged the two symmetrical sides together.
When adding the subdivishion Surface modifier and/or adding the smoothing Surface, I cannot get this edge to be smooth as the others. It forms into a sharp line all along my mesh.
I tried deleting doubles using Merge by distance and I tried smoothing it in sculpt mode. I tried deleting the edge by combining the two faces into one, but that just made two weird lines instead of one.
My blender file. I have no idea how to fix it.