"not in sync: resp=0x00" No forum solution seems to work

Hi everybody, I know that there are approximately a bazillion threads about the infamous avrdude: stk500_getsync(): not in sync: resp=0x00 error, but no one of the solutions proposed is working for me. Here's my story: I literally took out the Arduino UNO from the box and tried to connect it for the first time. The L led was blinking from the moment I connected with the USB cable. As soon as I tried to upload, the message popped out. I tried any single step in the Troubleshooting, I bought a new cable, I tried it on a pc with Lubuntu (16.04) AND two different Macs (El Capitan), I checked that Port and Board are correct. I tried to press the reset button, apparently the L led should blink fast, this is not happening to me (it stays on as long as I keep pressed, but gets back to the 1s blinking as soon as I release). I would be very thankful about any further suggestion. Cheers, Leonardo

Original or clone? It sounds like missing the bootloader and only default blink sketch loaded. I am guessing, it is not original Arduino.