Dan in support offered the following helpful information.
"First, lets make sure you're at the most current software
version and the correct market on the tool. From the home screen head
to Setting, check the Market and make sure it's set to America. After that head down to About. The Sys. Software Version should be at 2.85. Providing all that checks out try the following.
"Instead of selecting a 2020 Prius, try using the Corolla Hatchback selection. There's only one year option which is 01/2019-12/2019 (315Mhz).
This model uses the same OE part number sensors as the 2020 Prius. In
our tools the 2020 Prius option uses an updated part number that
superseded the original. Some cars don't gel very
well with that new number and can cause issues like the one you're
experiencing. As for the Corolla Hatchback selection, your Prius and
the tool won't know the difference.
"Using that you can perform your OBD relearns like
normal. Side note, our tools will display a position for a spare wheel
if it was an option for that vehicle to have a full-size, sensored
spare. If your vehicle doesn't have a sensor in the spare you can
bypass it by selecting the spare using the arrow keys, pressing the
trigger button as if you were reading a sensor, then hitting No (N
button) while it's attempting to read. That will cancel out that
position and allow you to proceed."
----- my results ----
My tool was already up-to-date. Selecting the Corolla Hatchback led to instant success in reading the sensors. After skipping the spare as instructed, I then performed the OBD2 relearn (writing codes to the car's ECU).
Shortly after that, all was well, and a short test drive was successful.