Q:

How to enable Apple Sign in for existing app? " Provisioning profile doesn't support the Sign In with Apple capability"

I am trying to enable Apple Sign in for existing app, but am getting "Provisioning profile doesn't support the Sign In with Apple capability" and "Provisioning profile doesn't include the com.apple.developer.applesignin entitlement." 
Following Apple docs and other sources I've attempted several things.   In Xcode I tried:

and then in the Apple Developer Account for our app I tried enabling Sign In for our App ID under the Identifiers section. 

So a major issue seems to be that I cannot designate an App ID as the primary app.   Something else recommended to try in docs is to create a Key that in the Developer Account, but I don't think this is required.  I attempt this like so:

I guess my immediate issue is how to resolve the error listed for my provisioning profiles in Xcode.   My app builds fine until I try to add the Capability for "Sign in with Apple".   But it looks like the reason that may fail is because there is no Primary App ID, and assigning one is disabled.  
Notice this Stack Overflow question which does directly address one of my issues, but the answers are not effective for me. 

UPDATE:
By turning on "Automatically manage signing" in the Signing & Capabilities section of my project in Xcode I was able to get different Provisioning profile errors than seen in the first pic, errors I was able to resolve by deleting the profiles, generating new ones, and modifying other settings in the project, and then was able to successfully turn on Apple Sign in as a capability.   If you have a better solution, feel free to provide an answer before Bounty deadline expires. 

A:

If you are enabling it for existing app and none of the other Apple IDs in your developer portal is enabled with Sign In With Apple, can only enable it for primary App ID.  I think you got confused with the UI, when you were trying to Edit your App ID Configuration and checked Sign In With Apple you don't have to click on Edit, just need to scroll up to the page and click save.  
Because it's your first app id that you are going to enable this feature.  It will always be Enable as primary App Id and yes UI is confusing and it will let you click on Edit button and then both options are disabled on the next page.  
Now Sign In With Apple is enabled for one of your app id, so if you try to enable it for any other app id in your developer portal and click on Edit button you will see both options enabled Enable as primary App Id and Group with existing app id.

