Error saving Identifier capability change

I have an "identifier" for my app in Certificates, Identifiers & Profiles.. I am trying to change the "Sign In with Apple" capability for the identifier. I click "Edit" (see image 1) to open the "Sign In with Apple: App ID Configuration" modal.

In this modal, I change the radio button to "Group with an existing primary App ID", and then select my App ID from the dropdown; (see image 2).

I click "Save" on the modal and it closes as expected. Then I click "Save" on the main page, and then "Confirm". After clicking "Confirm", I get the error message below: "There is a problem with the request entity. Please choose a different app identifier to set up related consent." (See image 3)

I tried do the same process for another app and its identifier, but encountered the same problem.

Can anyone point me in the right direction? I've been in contact with Apple support but haven't made any progress in weeks. Thanks. My goal is to enable Sign In with Apple in my app; maybe my approach is wrong, but I saw somewhere that I needed to enable 'Group with an existing primary App ID'.