Warning light for transmission came on after jump start

Hello everyone. I bought my 2008 Prius in January with 175k miles on it and have been loving it every day. I plan on upgrading to the newer gen 4 next year but would like to keep this one as long as possible. I drive about 350 miles a week on average and last weekend I took it on a 400 Mile road trip. I live in Texas and most of the interstate speed limits are 65-75mph so I'm often going over 70. I know it's a Prius and it's not suppose to have a lot of power but it really appeared to struggle towards the end of the trip. I drove it only a few times while on vacation. I turned on my Prius for about an hour to charge my phone (didn't start the engine, just hit the power button without pressing the break). I've done this several times before with no issue before but the next time I went to start my car the battery appeared completely dead. The trunk would not open, I had to open manually from the inside and jump start it. Almost immediately after connecting the cables the car starts by itself. A red triangle warning light came on and an error message comes on the screen every time I start the car and when I'm driving it and come to a complete stop. The message reads "there is a problem with the transmission "P" lock mechanism. Park your car on a flat surface, and fully apply the parking brake." I've heard that having a faulty 12 volt battery could bring up false error messages. My Prius starts up and appears to be functioning properly. I'm still familiarizing myself with the mechanics of the Prius and I don't know much about the CVT transmission. How often should I get it serviced? How reliable are they? Also I popped the hood and skimmed the owners manual and didn't see anything about how to check or replace transmission fluid. Any help would be greatly appreciated. Thanks for reading

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Thank you everyone. An associate at my local auto part store helped me out with the light. He just put the car in neutral and applied the parking break and the light came off.
I may have just left it on a little too long but I'd like to get it tested just in case.
Suggestions on where to get the battery checked please...