It is helpful!
You get this error when you haven't selected a board in the Arduino IDE (FQBN is an acronym for "fully qualified board name"). So the only thing you need to do is select your Arduino board from the Arduino IDE's Tools > Board menu.
If you don't see your board on that menu, it probably means you need to install the boards platform for it via Boards Manager (Tools > Board > Boards Manager). If you have trouble with that, you can tell us which board you have and we'll provide detailed instructions.