Electrostatic Actuation of Cantilever beam

I,m trying to do the tutorial electrostatic actuation of cantilever beam, although i have followed exactly all the instructions but I receive the following error message. (I also checked all the physics setting and every thing against the already built model in the models/sensors folder)

here is the error message

Failed to find a solution for the initial parameter.
Segregated group 3

Singular matrix.

There are 400 void equations (empty rows in matrix) for the variable mod1.V.
at coordinates: (0,0,1e-006), (0,9.68925e-007,1e-006), (0,1.93785e-006,1e-006), (0,2.95474e-006,1e-006), (0,3.97164e-006,1e-006), ...
and similarly for the degrees of freedom (empty columns in matrix).
Returned solution is not converged.