macOS 10.13 has implemented an additional security layer for kernel extension drivers, such as the Line 6 audio drivers. After the Line 6 device is connected, the customer must open the Security Control Pane and accept the Line 6 driver in order for the device to be recognized.
Typically, you'll see this message when your driver gets blocked:
However, sometimes the driver gets blocked and the message won't appear. Your driver and hardware simply won't work. In either case, you'd need to approve the driver in System Preferences > Security & Privacy:
Additional information about this change can be found at https://developer.apple.com/library/content/technotes/tn2459/_index.html#//apple_ref/doc/uid/DTS40017658