Resignation policies

Hey guys! Recently I've received an offer from a startup based company. But they're asking to join within 30 days. And regarding my current situation in project, I've joined 1 week ago and haven't even got any access to start working in my project. If I get released from my current project and go on bench, I can ask for early offboarding. But my manager is asking me to serve 90 days if I drop my resignation. What should I do? If I resign, what's the policy to revoke my resignation and is there any cooldown period if I want to resign again?