There is a main issue we see in regards to the Prius (Gen 2, Gen 3) and the warning message "The Transmission P lock mechanism is Abnormal" that is displayed on the HUD (Heads up Display). If you check codes, you may see P3102 Transmission Control ECU Malfunction and/or C2300 Actuator System Malfunction.
The culprit is most often a bad or low charged 12v Auxiliary battery located in the right rear on the trunk. Here are a few videos that show the different symptoms a dead or dying Aux battery will exhibit...
Now if you want to replace it, we suggest that you use an OEM Toyota Battery from your local Toyota Dealer and watch this video to get an idea as to what it takes....