Yoon_sun 3.6.4 Tutorial Sensing Issue

Hi, I am doing the 3.6.4 Tutorial for Sim4Life 6.0. I got to the Simulation step for the Sensing part of the model. I cannot get the S-Electrode1 and S-Electrode 2 to go into the Sensing Voxel> Electrodes 1 folder. When I drag from Boundary conditions> Cathodic> S-Electrode1 to Voxel >Electrodes 1 it says "Copied" but does not appear in the folder. Could you tell me what I might have done wrong?