Error message: One or more of the CNAMEs you provided are already associated with a different resourceError message: One or more of the CNAMEs you provided are already associated with a different resource
Deleting the Amplify app, reseting Route53 Hostzone, reconfiguring DNS records all do not work
The list of apps you see in All Apps does not display apps that exist across all AWS regions.
Meaning, you could have an app of identical name existing in another region than the one you have selected.
Because that app exists, it is taking up the CNAME record which is pointing to the Hosted Zone of the same name.
Since Amplify automatically maps the Hosted Zone's name to the App Name, you likely have identical app existing in another region.
Visit other regions and delete the redundant app.