Please provide more details about your setup. Are you using a nRF52840-DK, and in that case, which of the USB ports are you connecting to? Is it the main one that connects to the Jlink debugger?
When the LD5 toggles continuously it usually means there is a problem with the USB connectivity. I suggest you try with another cable and see if that helps. Just make sure the cable supports data transfer and not just charging.