Start Host Debug Server

start_server.py

#!/usr/bin/env python3
import os

cmd = "litex_server --uart --uart-port=/dev/ttyUSB0"
#cmd = "litex_server --jtag --jtag-config=openocd_xc7_ft232.cfg"
#cmd = "litex_server --jtag --jtag-config=openocd_xc7_ft2232.cfg"

os.system(cmd)

We need to start a host debug server , 
depends on the debug interface.
You can use UART, Jtag, ethernet, SPI....

Reference 
https://github.com/enjoy-digital/litex/wiki/Use-Host-Bridge-to-control-debug-a-SoC