How to generate c form

HI All, 

My question may be very basic, coz my knowledge on online VAT compliances is very little so kindly help. 

I have a client in Bangalore who made some purchase from Chennai. Invoice date is Feb 4, 2017. The seller has levied 5% CST in the invoice already and requires a C Form for levying 2% CST. The client is yet to make the payment and has come to me for generation of C form. 

Based on my reading, I revised the VAT 100 return for the quarter Oct to Dec 2016 so that I can update interstate purchase on the VAT portal. Now, when I enter the invoice details in the field "interstate purchase" with the invoice date and details and submit, I get a message saying that "the entered interstate purchase can be approved in the next month only"

Am I doing this procedure correctly? Please guide me where I am going wrong.

Best regards. 

Aditi Bhardwaj