Google Play Internal Test "Item Not Found"

Question

While I can access the app in the Internal Testing track on my own phone, testers on devices associated with their registered email addresses receive an “Item not found” error. I’m experiencing the same issue in Closed Testing as well. I’ve tried almost every solution, and even attempted to upload the APK in the Internal App Sharing section, but it tells me “You must publish the app” there. I’m not supposed to publish the app at this time. Am I doing something wrong?