Arduino Uno First Program
Welcome to the Stack Exchange and on the Arduino World.

Just a note: Since yesterday there is a new version of Arduino IDE (the last version is now 1.8.11).

When you buy an Arduino, a default sketch is already present. This will blink your LED (pin13) with a 1 second interval. This just mean that your Arduino is working and has a bootloader.

To program it, you need to know which COM port is created for your Arduino module:   


Press WIN-'R' (execute application)  
Write devmgmt.msc


Check the COM port to see wich COM port was used for your Arduino.

This is the only cause, the message could appear if you use a standard USB-cable connected to your standard Arduino UNO board.