For ESP8266-based boards before using a tool like ampy you might need to disable debug output on the board. If you're using an official release build of MicroPython (i.e. one that ends in a simple version like 1.8.3 instead of a more complex daily build like 1.8.3-38-gf2a21a2) debug output is already disabled and you don't need to do anything extra. However if you're using a daily build or custom build from source you'll need to disable debug output that can confuse tools like ampy.
To disable debug output connect to the board's serial REPL and run the following commands:
esp.osdebug function should run and return no output. After running the command debug output will not be printed to the serial terminal and you can use tools like ampy.
It is highly recommended to add the above two lines to the board's boot.py so debug output is disabled permanently. If you don't make this change you'll need to manually disable debug output every time you reset the board! You can learn more about the boot.py file on the Boot Scripts page of this guide.