you need to be on the last part of her quest and you have to have maintained her innocence also it requires you to have bathed and head pet her sufficiently to unlock
that isn't enough to break sleep with cocona if you used option 2 when talking to cocona afterwards you are on the whore route otherwise cocona still innocent although if you don't teach her about prostitution when cocona catches you prostituting then it will default back to the innocence route again basically if you get coconas second CG thats the point of no return