The board must be completely unpowered. Press and hold the button while connecting the USB will cause the board to go into USB ROM mode. In this mode, it can receive boot information over USB. If it does not register a device in lsusb, the board has either booted or had residual power elsewhere and failed to go into USB ROM mode.
From USB ROM mode, pyamlboot sends the board the bootloader over USB. The board will boot into u-boot and then run a script that calls ums 0 mmc 0 which turns the device into a flash drive.
Alternatively, you can flash the eMMC using the instructions here: Libre Computer eMMC Flash Tool (LEFT) for Rapid OS Deployment