Importing .STL into Salome

I have an STL file that was generated using Rhino7. I succeeded in importing the file into Salome (Shaper), and I transferred it to the (geometry) module, but I am not able to define groups in this model. When the dialogue of creating groups is displayed, I cannot select the model in the text field (Main shape). How can I define groups of such a model in Salome? thanks for your help

Hello,

Can you provide the stl file , so we could take a closer look?

Mohamed