When I place the bateries inside the compartment, the camera starts working as if it is taking a photo. The roller is...

@Anahi Gutierrez "S means it’s on selfie mode" = NO "S" stands for START... the first time you take a photo it will release the plastic cover... that is why there is an "S" before the 0 (ZERO) *** ***

open the compartment for the film... you will see that in the middle of the right there is a small silver piece of metal.... if you press it in, the numbers will count, if you do NOT, it will return to "S" So now look at the door is the piece of plastic that presses that button still there, did it chip off?

Here is what I found when taking the instax mini apart: https://youtu.be/Jj4bJowzIuo