Please provide details of your project.
- What is the project about; full details please so we can try to judge what can possibly be wrong or possibly has gone wrong.
- Did it work before? If yes, what changed?
- What is connected to pins 0 and 1? Disconnect as it will interfere with uploads.
- Anything else connected to the board? Disconnect it and try again.
- Original Uno or clone? For clone, which serial-to-usb converter is on the board? If the serial-to-usb converter is not the CH340, run the loopback test and post the results
- Correct board and port selected?
- What does your operating system think of the board?
- Does the selected port disappear from the IDE if you disconnect the board?
Possibly less relevant but good to know
- Which operating system?
- Which version of the IDE?