You are planning international travel from Mumbai right? If yes, you don't need a negative certificate to enter the airport, if you are fully vaccinated. Carry your vaccine certificate they might ask.
You Do not need RT PCR to enter the airport.However, If the Airline You are traveling with or Your destination country needs RT PCR Then you would need to have One and it will be checked at counters during check-in.So check your Airline requirement as well as your country requirement well ahead of your departure.