Possession Is Supposed To Get By Dec 2019 But Still OC Not Received

I have bought a flat under construction in Thane in Sept. 2017. The builder promised us that we will get possession by mid of 2019 whereas in agreement, it is mentioned as Dec-2019. I was fine with that. But now it's been Jan-2021 and the builder keeps on saying that he has still NOT received the O.C. and hence can not provide the possession. In the beginning of 2020, I was convinced that due to COVID-19 the possession got delayed but even after unlock which starts in Jun-2019, I'm following up with them for the possession and in each month they are promising that by end of current month they will get OC. While booking the flat, we were conveyed that, if the possession date gets delayed, according to MAHARERA rules, builder is liable to provide 1% of Agreement value as penalty in each month but I do not know whether it's true or is it true then how to obtain this I'm confused how to process further. Please suggest