how to plot s(2,1) in HFSS for microstrip patch antenna

Go to left part of the window in project manager, right click on results, select "create modal solution data report"-> select "rectangular plot"->select "s parameters"..
From there you can select the appropriate port parameter and click on "new report"

Why would a single microstrip patch antenna have two ports?

thats right ... but without s21 how permittivity and permeability plots can be obtained for metamaterial antenna...... so that negative refractive index can be verified

I would test the metamaterial independently of the antenna. What kind of metamaterial are you using?

means you are saying that first test the unit cell of metamaterial structure for negative refractive index.I am using circular split ring resonator. How the metamaterial structure can be tested independendtly please tell

I'm guessing the SRR is etched in the ground plane (?).

In this case, I would consider a normal incidence test. Excite a plane wave normally to the SRR and observe at which point the reflection phase is ~0 -- while not strictly correct this method is used all the time in the literature.

Alternatively, observe the transmission properties through the SRR when illuminated with the same plane wave (you can get the S21 from this process). Of course, this implies a surface, not a material, but again the difference is usually glossed over in the literature.