Possession delayed by 9 months

Hello Sir/mam We have purchased under construction flat in Feb 2018. Builder mentioned possession date as Dec 2019 in sale agreement with no grace period. In Rera registration completion date is may 2021. Flat Is in pune municipal Possession in Dec 19 delayed and then he said he will give possession in next 3 month but then corona started and we are still waiting for possession. Work of building is about to complete but possession is delayed by 9 months After corona he mentioned that possession will be in Jun 20 but again that is not done. He has sent this on WhatsApp. We are staying on rent and he told possession will be there and you can give notice to owner of rental property but he failed to give possession. We have tell flat owner that we are extending it. It is very frustrating Can we file complaints in Rera for compensation and mental trauma due to delayed possession?