You should NOT see this error if you are using Unimesh! If you are using Unimesh and you see this, you probably accidently moved your entire mesh or something.
To fix this error, import another mesh, to get a reference for a correct neck edge. Then use extended manual edit to copy over the vertex and normal values. Or move the reference group to the top of the list and use Unimesh Vertex Data Merge and Normal Data Merge to copy the data. (then delete the reference mesh)
Also, if you tried to make a teen body fit an adult by simply changing the values in SimPE - you will have a gap, because the teen body will not meet the adult's head. You must actually create a new adult mesh file and bring in whatever mesh parts you need and edit them to properly fit an adult skeleton (and match the adult's neck) to have it work correctly in the game.
Long ago we used to get this error due to what seemed to be a rounding error in Milkshape, and simply moving the vertices up a tiny bit mostly fixed the problem. If you are not using Unimesh, you might try this if you have a very small gap.