If it's a low oil problem.
Be sure the motor is idling while you check the transmission fluid, since it's an automatic on works on a umping system it needs to idle. If that is indeed the problem I suggest draining the oil, check if the vehicle has a oil sump pan on the transmission. Open that up and replace the filter/strainer if you can see it without needing to remove the transmission.
I hope it's not a serious problem for you though. Have a good one!
!BEER