You should certainly still be able to amend your MO state return if you learn that you are eligible for a rebate/credit that you failed to indicate when filing your return. You'll just need to amend your state return.
See the linked information below. Specifically, watch the video included on this page which will clarify your eligibility.
https://dor.mo.gov/personal/ptc/
*** I am NOT a tax expert. I am a seasoned TurboTax user, and volunteer to provide assistance to TT users. Nothing I post is to be considered TAX ADVICE; I bear no legal liability for responses.***