Ok, I've done that one twice without failing it, so have no experience with that scenario unfortunately. It may be bugged, only thing I can recommend is rebooting your console and bringing the save back up and see if the mission is available. Outside of that, wait for someone else to respond with other ideas, or last case scenario, start a new game
![Sad :(](./images/smilies/icon_e_sad.gif)
Sorry I was not more help, good luck.