com.android.builder.testing.api.DeviceException: No online devices found.

The error message "No online devices found" indicates that the Android Debug Bridge (ADB) is unable to detect any connected or running devices or emulators. Here are some steps you can take to resolve this issue:

1. Verify USB Connection: If you are using a physical Android device, ensure that it is properly connected to your computer via a USB cable. Check if the device is recognized by your operating system.
2. Enable USB Debugging: On your Android device, make sure that USB debugging is enabled. To do this, go to "Settings" > "Developer options" (or "Developer settings") > Enable "USB debugging". If you don't see the "Developer options" menu, you may need to enable it first by going to "Settings" > "About phone" > Tap on "Build number" several times until it says "You are now a developer".
3. Restart ADB Server: Restart the ADB server to reset the connection. Open a terminal or command prompt and run the following commands:

   adb kill-server
   adb start-server

   For Windows:
4. adb kill-server adb start-server
5. For macOS/Linux:
6. Check ADB Devices: Run the following command to check if ADB recognizes any devices or emulators:This command will display the list of connected devices. If your device is properly connected and detected, it should appear in the list.
7. adb devices
8. Restart Emulator: If you are using an Android emulator, close the emulator and relaunch it from the AVD Manager or the command line.
9. Update ADB and Drivers: Ensure that you have the latest version of ADB installed. You can update it through the Android SDK Manager. Additionally, make sure you have the appropriate USB drivers installed for your device.
10. Try Different USB Port/Cable: If you are using a physical device, try connecting it to a different USB port on your computer or use a different USB cable.
11. Reboot Computer: In some cases, restarting your computer can help resolve connectivity issues.
12. Use Different Emulator: If you are using an emulator, try using a different emulator or create a new emulator instance using the AVD Manager.
13. Check Firewall and Antivirus: Verify that your firewall or antivirus software is not blocking the ADB connection. Temporarily disable them to see if it resolves the issue.
14. Try on a Different Computer: If none of the above steps work, try connecting the device or running the emulator on a different computer to determine if the issue is specific to your development environment.

By following these steps, you should be able to resolve the "No online devices found" error and successfully connect your device or emulator with ADB.