I'm working on AutocCAD 3D 2016 and I drew a solid with a void space inside. The image below is an example of the solid (actual solid is much more complicated, but topology is the same).Now I want to fill the void space with another solid, but I couldn't find commands to do that simply.Is there any solution?

Yes, UNION would be one option.  INTERFERE + UNION would be a second option.  However, without an actual copy of the drawing to view and test it is difficult to say for sure.Upload a copy of the drawing to a file sharing website like DropBox and post a link to it here.  Someone will take a look at it and get back to you.
John Connor2015-10-02 11:47:43

I assume from the word "another" that you want the Solid that fills the void to be separate from the one with the void in it, in which case UNION is not what you want.If it's not too complicated to make another Solid exactly like the one shown but without the void in it, you can then make a copy of the one shown, and SUBTRACT that copy from the no-void version, and the result will be another Solid filling the void.

Thank you for your advice! The problem has been resolved.I made the copy of the solid without void by COPY and SURFSCULPT.Then, I used SUBTRACT. It worked perfectly.