Attempting to purchase an in-app purchase (to buy coins) in Minecraft on an Amazon Fire Tablet I was constantly being told that my Amazon password was incorrect.
The issue turned out to be down to OTP being enabled on my Amazon account, using Google Authenticator.
To resolve the issue, it involved entering the password, with the OTP added immedately on the end.
If OTP is 345678, and password is: MyPassword!
Then you’d enter: MyPassword!345678