What is the minimum amount required to buy call option

I tried to buy call option for Nifty 50 Oct but it was rejected.

As per zerodha margin calculator below the margin required to buy the call option is zero. And the price of the call option mentioned above is around 340 rupees.
https://zerodha.com/margin-calculator/SPAN/

And I have around 3k in my zerodha account.
I am not selling the option, which would require a huge premium.

So wanted to know the reason why the order got rejected.

Lot size = 75

You need minimum 75*340 to buy one lot which is around 25K

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Thank you for your reply