Unable to do tetrahedral mesh for combined sphere

Hi,

I constructed a combination of two sphere. Initially, I faced some difficulty to combine two sphere using union but eventually I manage to do it by splitting it. 

In the first step, I initialized two spheres separately. I just combined them by changing the coordinates of the two spheres. 

Then, I used split command for both of the spheres and use union to combine all the spheres.

  

<?xml version="1.0" encoding="UTF-8"?>

 

 

After the split and union process, I set the region for both sphere as dielectric medium and set for FEM computation. However, when I wanted to create mesh, there were both triangular and tetrahedral mesh though I wanted only tetrahedral mesh. 

I also received the error message as shown below. 

 

Please help as I am not which part I have done error. I am not whether the error occur at the split, union process or in the initialization of the dielectric medium for the spherical region.

 

Thank you