Code checking before programming

Projects General Guidance
2

unsigned char encoder_A1;
Could be
byte encoder_A1;

const int pin_A1 = 2;//A pin red encoder
Could be
const byte pin_A1 = 2;//A pin red encoder

int brightnessRed = 120; //Starting at half the value of full brightness
Could be
byte brightnessRed = 120; //Starting at half the value of full brightness

Depending on the wiring, you may be able to turn on pull-ups.

You will of course need a loop() function.

You need to get the better half buy your child a soldering iron, DVM, hand tools, drill etc.