How to coarse grain several H2O molecules into one DPD bead?

Dear All
       I am facing a question about the operation of defining several H2O molecules as one DPD bead. My operations  are as follows.
       With MS 7.0, I use "Build" module and choose"Build mesostructure" => "coarse grain". Then I choose "Motion groups", not "Patterns" or "Subunits". The next step is click the "More" => "Create(from selection)". Of course I have already use Amorphous Cell to construct a box with several H2O molecules and made all molecules be chosen before I use "Build" module.
      After I click "Create(from selection)", the result is show as images. 


But when I clicked "Build" button, an error happened.


So, what are the right steps to coarse grain several H2O molecules into one DPD bead?
Hoping for help.
Thamk you all.

Best regards
Fang