Mass-effect – Cannot Access the Rescue Admiral Korlis Mission

You'll get this mission at the same time you get Rannoch: Geth Fighters. If you haven't advanced the plot to this point, you won't have this mission. This happens after you've assaulted the Geth Dreadnought.

Note that it is important to play the Admiral Koris mission, and the other side mission on Rannoch (Geth Fighters) before completing Priority: Rannoch, as it's possible that the outcome of Priority: Rannoch will be influenced by whether or not you've completed these two missions. Completing either of these two missions will unlock access to Priority: Rannoch.

It's the Shadow Broker Wet Squad war asset, and it's located on Rothla in the Dranec System. You'll have to scan for it and then probe the planet. Make sure to go back to Barla Von before starting Priority Tuchanka.