how to start a "hello world' and connect the board to my mobilephone

I turn on the device, the led turns green. And I use usb to connect it with computer then I  open the nrf connect for desktop, the bluetooth low energy tool,

Received status with code 0 PKT_SEND_MAX_RETRIES_REACHED, message: 'No response from device. Tried to send packet 6 times.'
serial port COM3 closed.
Error occured when opening port. Errorcode: NRF_ERROR_TIMEOUT (0xd)
then this error happened, and I cant find the device on my phone.
How can I start a hello world with my phone?