Problem

Using Automatic Signing in Xcode 15.4, I received a “Failed Registering Bundle Identifier” error when I went to go add a new Capability for one of my Targets:

While the error message states:

The app identifier “…” cannot be registered to your development team because it is not available. Change your bundle identifier to a unique string to try again.

the app identifier (Bundle Identifier) was already registered under my account in App Store Connect, and signing was previously working prior to adding a new Capability (which updates the Provisioning Profile).

Solution

The solution was to prefix my Bundle Identifier in Xcode with my Apple Developer Team ID (which can be found at https://developer.apple.com/account under Membership Details).