This is my first printer build Trying the flash the board and I'm getting this for my kossel. I have had the thing built almost a week now and been new at all this I'm finding it hard to get going. The lcd has black squares across it, nothing else. Got it to do the blink test✅ And that's as far as I have gotten. Can anyone tell me what this is or how to overcome the problem please..

Did you set up the com port on your pc?
Did you configure Arduino properly to use that com port, board and processor?
Did you connect the LCD correctly (connector 1 to 1 and connector 2 to 2)?
Try to reset board and LCD by pressing the micro switches
Try to flash the board with nothing connected to it but the USB. At first the 2 blue leds are blinking slowly, while flashing they blink quickly and after a successful flash they're off.

Thank you for the reply. I have followed all your suggestions and rest. Still getting errors and a blank screen. Any other ideas, I'm all ears.