I just replaced the battery and the negative charge connector on my 2012 mazda 6 and the car still won't start. Does it have to be reset at the dealer?

I would guess that you have a bad connection at battery. You're getting enough current to power the little stuff but not enough to turn the starter. Maybe double check battery terminals.

My car engine light is on. I replaced the mass Air flow sensor and the light is still on

If your engine light is on have the trouble codes read and then post them in a new thread.

That's what the code said Mass Air flow sensor

Maybe you need to clean or replace the MAF. If that does not work Post a new question.

l have replaced the mass Air flow sensor

I would guess that the check engine light would reset itself on a 2012 model year. You might google how to reset CEL and see if that fixes. Did you replace MAF with a new or used? If used, it might need to be cleaned. O'Reillys will have the cleaner needed for the sensor.

with a new one

I suggest trying to reset light and see if that fixes. You can reset on a lot of cars by disconnecting battery negative terminal for a minute or so. I drive Toyotas and you can simply remove/replace EFI fuse to reset CEL.