Open On Arduino IDE, go to Tools > Port and make sure the right board, Arduino Uno, is connected to the right port (tty.usbmodem.* for Mac or Linux, cu.usbmodem* for Windows).
It usually is /dev/somenamehere (usually USB0 or ACM0) for Linux, and a COM port for Windows.
Just a small correction
It usually is
/dev/somenamehere(usually USB0 or ACM0) for Linux, and a COM port for Windows.You are right, thanks a lot for the comment :)