If you’re trying to set up DeviceCheck access, I suggest ignoring Apple’s certificate set up instructions at the start of the DeviceCheck docs. They’re garbage. So bad they’re not even wrong.

Instead:

1. Go to Certificates, Identifier & Profiles in the Dev Portal
2. Go to the Keys tab
3. Create a DeviceCheck key

Now you can continue reading the docs.

NB: You’ll need your Team ID from the portal, and the 10 character identifier attached to the Key you just created (which is in the portal, and the Key’s filename), for the JSON object you’ll be creating.