LG disabled fastboot commands on almost all North American and European retail models (e.g., LM-V450VM, LM-V450PM, LM-V500EM). If you try to enter Fastboot on these, you will likely end up in instead.
Your bootloader is locked. No workaround except unofficial bootloader unlock (not available for most variants). How to Open Fastboot Mode on LG V50 ThinQ
However, the LG V50 ThinQ runs on modern Snapdragon architecture where the term "Fastboot" is often used interchangeably with the bootloader interface. Depending on what you are trying to achieve, you might be looking for: LG disabled fastboot commands on almost all North