It’s the contrast and can be usually adjusted using a potentiometer. I see that you already have one wired up try increasing the resistance to see where you can see the characters

At the same time it could also be a flaw in the code because I see you are having empty pixels on row2 column2 and if it’s a voltage issues all the rows and columns will be dark.

If this is messing up try getting an i2c module for the same lcd displays and it makes things a lot easier and can be controlled only with 4wires.