I found the issue "Your Amazon Pay Balance Account is inactive. Please verify and re-activate your account."
When I try to do so, I am getting error. App is unable to verify mobile number. Tried all tricks, didn't work. Just raised a complaint with Amazon customer care. They will need 2 working days to check and revert.
P.S. Found similar issue related posts in reddit - Amazon customer care says device is blocked for activation and is not able to provide any resolution. After reading this I realized, mine was active but since I changed my phone around 6 months back it has stopped working. God knows if they will be able to resolve my issue or not.