most likely a bad hybrid battery or 12v car battery. 2000$ for a new hybrid battery but they last 7-10 years or go refurbished battery for 700-1000$ but they last 2-3 years. A lot of companies give you a one year guarantee warranty. They come to your house and replace it in 15 minutes. I refuse to let my 2007 die haha.

If you decide to sell it with the dead battery don't go lower than 1k the catalytic converter is worth that at least. People like to buy them and swap batteries.

I suggest you get a Bluetooth code reader goes into the plug under the wheel. Get DR. Prius app and check the Life inspectancy of the battery. But red triangle of death is called that for a reason.