How can I fix the error message waiting for device on my LG 4?

[AC Question]

lg4 waiting for device

i trying to unlocke lg4 bootloader
i need the id device so i try it by
adb reboot-bootloader
then
fastboot oem device-id
however when i try
fastboot oem device-id
its just stuck
i got <waiting for device >
in cmd
what i need to do or didnt do and if u can explain in detailes