Vastu Vihar — not handing over the flat booked about 3 and half years back

One of my relatives has booked a nano flat in the project of vastu vihar near aiims, patna about 3 and half years back. The full payment was done in october, 2019. The company has promised to allot flat and hand over the same just after the payment but till date after more than 10 months of the complete payment, they have not allotted flat. They said firstly the flat will be allotted just after deepawali in 2019, later, they ask saying they will provide by december, 2019 and after that they are till now escaping day by day by making forged promises and extending the day of allotment of flat day by day creating great inconvenience to my relative.