- Did it work in the past?
- What is the project about?
- Clone or original?
- What does your operating system (which one) think of your board?
- Correct port selected?
- If your board does not use the CH340 serial-to-usb converter, you can run the Loop-Back Test Instructions to analyse a little further.
- What is all connected? As mentioned above, disconnect. If you have anything connected to pins 0 and 1, that will interfere with the upload.