I did make a little progress early this morning. After including your change AND redefining the z parameters as

solution = NDSolveVlaue [{pde, boundaryConditions},   
                                 V, 
                                {x, 0, groundLength}, 
                                {y, 0, groundWidth}, 
                                {z, -0.1,  distanceToGround}];

I'm no longer getting the an from NDSolveValue Proceeding on with execution of the code below... the remainder of the code executes without error but unexpectedly does not produce any vector arrows!

electricField = -Grad[solution[x, y, z], {x, y, z}];
VectorPlot3D[ Evaluate[electricField], 
                      {x, 0, groundLength}, 
                      {y, 0, groundWidth}, 
                      {z, -0.1, distanceToGround }, 
                      VectorScale -> {Tiny, Scaled[0.02], None}, 
                      VectorStyle -> Arrowheads[0.02]]

Re: NDSolveValue::bcnop

Yes, I noted that NDSolve[] didn't work in the 2nd para. and suggested one might try to manually create an ElementMesh with the cuboids as subregions. When NDSolve[] complains about not finding points on the boundary, I think of two things. One, it mishandles the solid region. Two, round-off errors cause the problem.

In any case, I do not know whether generating your own element mesh would solve the problem. This tutorial, https://reference.wolfram.com/language/FEMDocumentation/tutorial/ElementMeshCreation.html, describes how to do it. If you need help, I suggest also asking on mathematica.stackexchange.com, since this discussion has not attracted the sort of expert help you seek.

Re: No vectors

If I remove the options VectorScale -> {Tiny, Scaled[0.02], None} and VectorStyle -> Arrowheads[0.02], I get some vectors. The following shows that the norms of the gradients are quite small, with 3 having an exponent base 10 greater than -8 and most being between -9.6 and -8.6. The variation in scale may be causing the vectors to disappear in the plot. Or maybe it's because VectorScale has been superseded as of version 12.1; see the link. If that is the reason, it would have been polite for Wolfram to have programmed in an error message.

The following shows that V[x, y, z] seems to be a constant 200, plus or minus some numerical error.

Table[(solution[x, y, z] - 200)/200, 
                      {x, 0, groundLength, groundLength/4}, 
                      {y, 0, groundWidth, groundWidth/4}, 
                      {z, -0.1, distanceToGround, (distanceToGround+0.1)/4}] //
                      Flatten // Chop

I figure you will conclude that NDSolve[] is not working properly. But if it is, congrats!