As per HFSS v9.0, if you are using a 2D-sheet for a radiating patch or for a ground, then no material we can assign to it. Instead we can only assign a boundary to it, whether it was a Perfect E or a Perfect H; it depends upon your design.

To assign a boundary just right click after selecting a 2D-sheet, then go to Assign Boundary and select the one that suits your design.

Dear Raj, you can apply a "Finite Conductivity Boundary" on your 2D sheet, which will allow you to specify the material and thickness. Consider that this boundary works well only if the thickness is much larger than the skin depth in your frequency range.

thank you @ suresh sahni and @wolfgang amadeus . 

in simulation your answers are okay.. what about fabrication process . what they will fix thickness of sheet ... 

Regarding the fabrication process, the choice of the metal thickness really depends on the application and the range of frequencies. A good rule of thumb consists in having a thickness around 5 skin depths at the frequency of interest. Above that, the improvement in performance becomes negligible.

thank u  ....so much sir 

I think the best way to decide if a "finite conductivity boundary" works as a very thick box is comparing two canonical models. If the results agree well, you will probably decide to simulate your problem using a finite conductivity boundary (2D) assigning a thickness instead of using a thick box, because HFSS resolvs the first case faster.

thank u @Leticia Alonso González

Hi all, I designed an wearable antenna which the patch is curved. the patch as 2D and has no thickness. Can I use PEC boundary condition on curved sheets?

@Amir Arayeshnia

yes you can, in the same way as assigning PEC to the regular sheets

Rclk on it  > assign boundary > perfect E