To get the Run Bitzen! Achievement in Bitlife, you will need to bet on the horse called Bitzen and win at least once. That being said, to do this, you will first need to age to the point where the activity, horse racing, unlocks for you.
It will unlock more or less when you finish high school, but you should probably get a decent job before heading any further. This is because horse racing can suck up a lot of money fast, and you will need a considerable amount of money to afford bets.
Related: How to win horse races in Bitlife
So we recommend getting a job such as a CEO or better to earn yourself huge amounts of cash. Once you got the right job, you will be able to place bets via the horse racing tab under activities. When you open the tab, you will need to first pay a 10 dollar admission fee.
After that, you will need to keep placing bets until a horse called Bitizen shows up - it won't be hard as he shows up quite frequently. That said, every time Bitizen shows up, place a bet on them, and sooner or later, the horse will win, and you will unlock the Run Bitizen! Achievement.