Just as an update to my previous posts. I had no problem installing the Walmart app on my new Pixel 2, running 8.1. And it works - I've set it up, the camera comes up, etc.
Evidently it's just incompatible, for one reason or another (it seems to claim to be incompatible with Elizabeth Carles' setup, but it works anyway) that has nothing to do with whether it actually is or not.
As far as fixing it, enable the apk mirror, download the apk from there and tap the apk file in a file manager (if you don't know how to install an app from an apk file - those who do can ignore this). But check the functions you're going to use before going to the store. Standing there trying to pay with an app that doesn't work can be a bit annoying. (No, you can't check out the paying part, but make sure it takes your CC info, and that the camera opens when it should.)