12864 LCD Screen Not Lighting Up
If your 12864 LCD screen is not showing any LCD backlight (you must have firmware loaded on your board for any text to be displayed on the LCD) then rotate the cables at BOTH ENDS but only on one side (aka LCD side or board side).
The wire with the red marking on it for each cable should be facing the same way (aka both left or both right, see pics in the link). For further details with pictures see here: https://www.th3dstudio.com/knowledgebase/12864-lcd-cable-connections-correct-and-incorrect-connection-pictures/
If you want a longer explanation on why this is needed sometimes see our video here: https://www.youtube.com/watch?v=BPl5-psFQRc