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This could be caused by various situations.
1. Make sure your target hardware is connected and matches with the current project.
2. If your device in the Arduino Nano 33 BLE Sense, try pressing the reset button twice before starting to flash.
3. Try connecting the device via another port of your laptop.
4. Check if your computer is connected to the internet through a proxy. If so, temporarily disabling the proxy may fix the issue.