Runaway master

Obtaining the pass
After learning what Fawn run away, go to the black market in the Slums. You likely already encountered the guy who sells junk; well, he also sells various counterfeit passes, each one costing you $100.

However, none of them would work; attempt to enter with the fake pass would result in you being arrested and fined for additional $250, while also losing the pass. There's one-time event if you previously finished the Serial pervert quest, but otherwise there's no reason to get caught.

Fixing the pass
You need to pay for another massage session with Lin-Lin; after finishing, she would reveal what your passes are too glaringly fake to work, but she knows someone who may correct them just enough to make it through: the local tattoo master.

Tattoo master would require another $500 for his work, but would make the job done.

From now on, you can create additional passes by yourself, by buying them in the Slums, and then, by tinkering with them via ink pen, correcting them to resemble the genuine pass. Be careful, though: in order to do so, you must pass the Nimble skill check, with difficulty of 15+(3*Nim)%. If you fail, the pass would be hopelessly ruined; on low Nimble, it may be more cost-efficient to just pay the tattoo master after all.

Entering inside
Now, when everything is in order, you may go into the UNA. Be sure what the following conditions are met:
 * You are not aroused;
 * You are not armed with weapons;
 * You are not dressed inappropriately.

Otherwise, you wouldn't make it inside.

Finding Fawn
That part of the quest is unfinished in the currently available version of the game, and can't be completed yet.