Builder asking to additional gst after paying all the amount as per agreement.

The builder is asking to pay an additional amount for maintenance and GST, but as per the agreement, I have paid the entire amount to the builder for 30 months. He has not given the No Dues certificate and Possession letter after paying all the amount.

Also, he has not constructed the approach road to the building for 2 years, and he is planning to hand over the society.

please suggest some solution.