Help with USB connection on Mac
This page contains help for when you have connected your Android device to a Mac with the USB cable, and the Connection Test says that the device is not detected.
You should not be here: according to Google's instructions in Setting up a device for development, "it just works." Of course, nothing "just works."
Try reviewing the connection steps at Connecting to a phone or tablet with a USB cable . If you can't find anything wrong, post an issue to the forum. But a problem at this level is unlikely to involve App Inventor per se. It's more likely a connection problem with the phone, perhaps even something as mundane as a bad USB cable.