Which iPad Pro 12.9" do y'all have? Notice there are three different images, since there are three generations:
Here, I have edited the choices to make it more clear:
HTH.
[Edit]:
FWIW, the instructions say you can either be on Mojave with Xcode 11 beta and install from iTunes, or be on the Catalina beta.
I used the Mojave + iTunes (with Xcode 11 installed first) method using the first image above on my iPad Pro 12.9" (2018 / Third Generation).
You have to know to hold the option key when clicking the "restore" button on the device view in iTunes. With the option key held down it will prompt you for the image. Sounds like many of the respondents got that far, but at the same time, many are not being very specific about which iPad Pro 12.9" they have, hence, my original reply.