I turn on my car and it starts, when i put it in reverse and i dont hurry and push the gas pedal the engine turns off. Whats wrong?

have you checked the air filter

Yes I change that out if it is dirty. Last one I change was about 2 months ago.

have you checked the IAC valve

If your check engine light is on, take it to Autozone and have the codes read. If no check engine light, remove throttle body, dismantle, and clean thoroughly with throttle body cleaner. Remove your Idle Air Control valve and clean that also. Remove your Mass Air Flow sensor and clean that with a can of MAF cleaner by CRC. Do not use throttle body cleaner on your MAF. This is the cheapest first step in fixing your issue. If after cleaning all components and issue still persists, post back. You may have to start replacing some parts. Just cleaning should run you about $20 for the spray cans. Here's a couple of vids... https://www.youtube.com/watch?v=XGJ91Eq2kGc https://www.youtube.com/watch?v=2Ojl7xL_c1Y

Could it have anything to do with the transmission fluid level?

No. Tranny fluid level has no effect on the engine.

You have a fuel/air issue. Cleaning the above mentioned components all have a direct effect on how fuel and air is delivered to the system. If your MAF is dirty, it will give too much or too little air causing a poor fuel mixture. Same with an overly carbon filled throttle body and IAC.

Check the rubber tube that goes from the air box to the throttle body. They get dry and crack. This lets unmetered air into the engine. It is a simple repair to replace it. I believe it runs around $80 for the part.

where is the Iac located on a 2011 nissan sentra