Mcu Unable to connect raspberry pi

[LifeOfBrian]

Are you sure about that:

[mcu]
serial = /dev/ttyUSB0

I replaced the board of my Anycubic i3 Mega before it was klipperized.
You might want to read here:

How-To Trouble-Shoot

How-To Troubleshooting The following is a collection of general troubleshooting tips. These should be followed before posting a Request for Help Basic Troubleshooting Connection Problems to the Printer Board Linux dmesg Works when cold, but stops working when hot or moving Note: Approximately 50% of typical problems are due to the items discussed below. Another 49.8% are configuration problems The last 0.2% could be either bugs or are just designed as is. Basic Troubleshooting Check your …

Especially the paragraph about Connection Problems to the Printer Board.