I'm trying to get my enlistment bonus, but my AIT certificate doesn't have my MOS code on it. What should I do?

Contact the school house and request a certificate with your MOS code.

Another document you can use is your DD 214 (that you should have received after training) stating your MOS training. If you don't have that, then take the advice of SSG(P) (Join to see) and contact your AIT school house. Because, you should also have received some sort of diploma/certificate of Regimental Affiliation.