You need to re-enable "Device Setup" and register your device before you can use the Kindle app.Kindle Reading App
I used the Toolbox app to de-Amazon my Fire HD 10 Tablet 2021, I used the recommended debloat option and just discovered the the Kindle Reading App was removed I tried enabling it but keep getting your app is not registered and when I try to sign in it goes back to the box saying my device is not registered. I used the Toolbox tool to add back the Newstand, Kindle Store but still no success same issue. A
Any Ideas, I'm sure I'm just missing a setting somewhere.