I recently customized my character (started as female) and clicked the dysphoria option in the Extras tab from the settings. I was able to hit the transition button under the personal tab once my character was 18+ years old.
From there I was able to rename my character as they transitioned into a male.
Make sure you've updated the app or reinstall it, and it should work. Perhaps it's a random bug that will need to get addressed where it works for some and not for others.