As a followup - after upgrading to Unity 6000.2.6f1, these errors seem to have disappeared. If you're running into these issues on an earlier Editor version, try upgrading if you can.

These issues plague my project constantly. I'm using Unity 6000.0.33f1, and the latest from the Apple Unity Plugins repo. I've confirmed my build of the plugin is successful, notarized, and targeting the correct platforms. Most days, I have to repeat "Build & Run" until eventually it gets through. It seems like there is a script execution order issue that is a 50/50 chance of the native Core & GameKit libraries being found, or perhaps an issue with the multiple build steps of Addressables. I have also disabled all other iOS post processing steps, including unity's built in Entitlement generation. This issue is a huge sore in iOS Unity development currently, when utilizing the official plugins.