I paid full amount for a flat in Kolkata but builder is not registering it what to do

You can file a suit for specific performance before the civil court in Kolkata, seeking a court order directing the builder to execute the sale deed. Under the RERA Act, 2016, you can also file a complaint before the RERA Authority in West Bengal for delay or deficiency in delivery of the flat. The RERA Authority can order the builder to refund with interest or complete the project within a specified time. Under West Bengal Premises Tenancy Act 1997, there may be additional real estate regulations applicable to your situation. If the builder is insolvent, you can approach the Calcutta High Court under West Bengal Premises Tenancy Act 1997 for appropriate relief.