Q: Keychain not filling up iPhone backup password

while troubleshooting an issue, i had to erase my iPhone and after that i am trying to restore from my recent back up.

Overtime the keychain used to autofill the iPhone back up password, and now its not. I am unable to restore the back up and its very sad.

I tried whats mentioned in  Lost Password to iPhone Backup but it doesn't work. Keychain access does not even show the iPhone backups.

Please help me to get my backup restored.