When you get the message...
'Error: No deformable objects selected'
It generally means that you've tried to create a blendshape node using geometry other than a duplicate of the original geometry or that you've deleted/added something to the duplicate while you've been adjusting it to make the new shape eg. verts, edges, faces.
You must keep exactly the same properties on the duplicate for the blendshape to work, you can't delete or add anything, you can only move the verts to create the new shape.