Can't upload any sketch to the arduino uno

Using Arduino IDE 1.x
9
  1. You're using an Uno R3. Is that an original or a clone?
  2. Is the board correctly recognised by your operating system?
  3. Which Operating system?
  4. Did it ever work? Or did it start failing because you did (by accident) do something to it? What was the last sketch that you were working on when it started failing?

If your board does not use the CH340 TTL-to-USB converter and is properly recognised by your operating system, you can run the loopback test.