Rob
Ansys Employee

Which solver are you planning on using? It may be you can model the balloon as a thin surface which may simplify things. The other issue may be if the balloon touches the cube as this will create a very poor mesh at that point, and it might be this that's preventing the thin volume from meshing. 


Try pasting images into the text as the attached files are unreadable here! This will also enable the community to give you some more pointers. 


Cheers,
Rob