Margin calculator does not work for options? I always see NA

You are trying to add a option buy position on the SPAN calculator. For option buying there is no margin requirement. Risk is limited only to the premium you pay to buy the option. Since risk is limited no margin is charged. Margin required is equal to the premium paid. So if you want to buy Nifty 8100 trading at Rs. 98, you'd require 98*75=7350 in your account. This isn't shown on the SPAN calculator.

If you try adding a short position, however, you'll see the margin requirements as shown below.

Here, you'd require the full total margin to hold an overnight position or 40% of the total margin for an intraday (MIS) position.