It’s not your Sync unit. Ford updated the authentication endpoint recently. Check with Amazon as they might have put up new instructions for connecting the vehicle thru the apps. Or just delete the app and reinstall and re-authenticate.
Not the car, the app. In the app you need to authenticate with Amazon. This needs to be the same authenticate for the Amazon devices in your house, and on your account. Next, disable CarPlay and/or Android Auto. When you master reset, these probably turned back on. Also, on your phone, disable these functions. Iphone is USB connection, Android is BT. With the app running on the phone on the main screen, turn on the ignition and add the app to the AppLink area. If all this works and the app is added and shows on the Sync screen, but does not respond to commands, etc., then there is an issue with the app and Amazon, not Sync. You will have to remedy that problem frst before this will work.