just to further expand on this. for the lightmap all objects must be unwrapped in the 0-1 space for both U and V and must not be overlapping. wrapping and overlapping are fine for other purposes (although not for baking normals).
and please mark this problem as answered if it is.
I was getting errors about “Object has overlapping uvs” on my meshes not having secound UV channels, I fixed that following the advice on here by creating or importing a second UV channel without any overlap. I then chose this light channel in the lightmass index, but after building lighting now the error has changed to “Object has wrapping uvs”. What’s the difference here?