Low transmission fluid can sometimes cause a no reverse situation but the no reverse situation on an automatic transmission could also be caused by a faulty transmission solenoid that is under the transmission fluid pan.
Your transmission has a few solenoids especially those electronic shift and when one or more of those solenoids fail on the transmission then you may have no forward or reverse on the transmission.
Other causes of no reverse could be the bands inside the transmission slipping or failing completely and sometimes the automatic transmission may need rebuilt to fix the no reverse.
I've had issues with transmissions that would not go into reverse and turns out it was a simple solenoid in the transmission pan.
But other times the transmission needed to be rebuilt especially if the transmission has a lot of miles on it such as in 200,000 or more miles.