This error indicates the port is open in some other application. Only one at a time is allowed to use the port. That could even be the Serial Monitor from another instance of the Arduino IDE if you have multiple IDEs running at once.
So if you have another application running that uses the port, make sure to close it.
You should also make sure you have the right port selected from the Tools > Port menu of the Arduino IDE. The port of the Pro Micro should be identified in that menu, but you can also use this procedure to make sure you have the right port:
- Unplug your Arduino board from the computer.
- Select Tools > Port from the Arduino IDE menus.
- Note the ports, if any, listed in the menu.
- Close the Tools menu.
The ports list is only updated when the Tools menu is re-opened, so this step is essential. - Plug your Arduino board into the computer.
- Select Tools > Port from the Arduino IDE menus. - The new port listed in the menu is the one for your Arduino board.
If you can't find another application using the port and you have made sure the port selection is correct, try restarting your computer. In rare cases, the port can get stuck in an unusable state and restarting the computer fixes this.