Wow, it works! Thank you so much Hans! Now I'm going to figure out how to replace the effects with other existing ones out there. Specially the Cyan or Rainbow March. 😀 

Welcome to the forum!

When looking at your code, I see a few things that may not be entirely correct (partially my fault maybe).
I've corrected a few things, and hopefully this helps you get to the next step 😊 

// MultipleStripsInOneArray - see  https://github.com/FastLED/FastLED/wiki/Multiple-Controller-Examples  for more info on
// using multiple controllers.  In this example, we're going to set up four NEOPIXEL strips on three
// different pins, each strip will be referring to a different part of the single led array

#define FASTLED_INTERNAL // just used to mute the Pragma messages when compiling
#include <FastLED.h>

#define NUM_LEDS_STRIP_A 121
#define NUM_LEDS_STRIP_B 30

#define PIN_STRIPA 6
#define PIN_STRIPB 5

#define NUM_LEDS 151

CRGB leds1;
CRGB leds2;

int Strip1Position = 0;
int Strip2Position = 0;

void setup() {
  FastLED.addLeds<WS2812B, PIN_STRIPA, GRB>(leds1, NUM_LEDS_STRIP_A).setCorrection( TypicalLEDStrip );
  FastLED.addLeds<WS2812B, PIN_STRIPB, GRB>(leds2, NUM_LEDS_STRIP_B).setCorrection( TypicalLEDStrip );
}

void loop() {

  meteorRain(0xff, 0xff, 0xff, 10, 64, true, 30);
}

void meteorRain(byte red, byte green, byte blue, byte meteorSize, byte meteorTrailDecay, boolean meteorRandomDecay, int SpeedDelay) {
  if (Strip1Position == 0) {
    fill_solid(leds1, NUM_LEDS_STRIP_A, CRGB(0, 0, 0));
  }
  if (Strip2Position == 0) {
    fill_solid(leds2, NUM_LEDS_STRIP_B, CRGB(0, 0, 0));
  }

  // fade brightness all LEDs one step
  for (int j = 0; j < NUM_LEDS_STRIP_A; j++) {
    if ((!meteorRandomDecay) || (random(10) > 5)) {
      leds1.fadeToBlackBy(meteorTrailDecay);
    }
  }

  for (int j = 0; j < NUM_LEDS_STRIP_B; j++) {
    if ((!meteorRandomDecay) || (random(10) > 5)) {
      leds2.fadeToBlackBy(meteorTrailDecay);
    }
  }

  // draw meteor
  for (int j = 0; j < meteorSize; j++) {
    if ((Strip1Position - j < NUM_LEDS_STRIP_A) && (Strip1Position - j >= 0)) {
      leds1 = CRGB(red, green, blue);
    }
  }

  for (int j = 0; j < meteorSize; j++) {
    if ((Strip2Position - j < NUM_LEDS_STRIP_B) && (Strip2Position - j >= 0)) {
      leds2 = CRGB(red, green, blue);
    }
  }

  FastLED.show();
  delay(SpeedDelay);

  if (Strip1Position < NUM_LEDS_STRIP_A + NUM_LEDS_STRIP_A) {
    Strip1Position++;
  } else {
    Strip1Position = 0;
  }

  if (Strip2Position < NUM_LEDS_STRIP_B + NUM_LEDS_STRIP_B) {
    Strip2Position++;
  } else {
    Strip2Position = 0;
  }

}

I don't have any experience in ws2812b projects using Uno, especially coding. I am doing my best to read/watch stuff about it online. So far, Tweaking4all has been my guide to start my project. From the materials needed up to making it work, this has helped me a lot. Now I'm on the part where I want to add a code for it. I tried to compile 2 existing codes for my project, The first part of the code was from arduino forums and the second one was from Hans (Meteor Rain). Although I really want a combination of Cyclon and Ranbow March, I think it may be advanced for me so I am doing a single effect for the 2 strips for now.

What I am using:

Arduino Uno & WS2812b (2 Strips / 30leds and 121leds)

I am getting this error message: expected unqualified-id before numeric constant  from the 2nd line:

#define NUM_LEDS_PER_STRIP_A 121
#define NUM_LEDS_PER_STRIP_B 30

I know more errors will come after this one. I just want to know what caused this issue to happen.

The entirety of the code that I was trying to compile:

// MultipleStripsInOneArray - see https://github.com/FastLED/FastLED/wiki/Multiple-Controller-Examples for more info on
// using multiple controllers.  In this example, we're going to set up four NEOPIXEL strips on three
// different pins, each strip will be referring to a different part of the single led array

#include <FastLED.h>
 
 #define NUM_LEDS_PER_STRIP_A 121
 #define NUM_LEDS_PER_STRIP_B 30

#define NUM_LEDS 151

CRGB leds;

void setup() {
  FastLED.addLeds<WS2812B, 6>(leds, 121, NUM_LEDS_PER_STRIP_A);

  FastLED.addLeds<WS2812B, 5>(leds, 30, NUM_LEDS_PER_STRIP_B);

int NUM_LEDS_PER_STRIP_A = 0;
int NUM_LEDS_PER_STRIP_B = 0;
}

void loop() {

  meteorRain(0xff, 0xff, 0xff, 10, 64, true, 30);
}
 
void meteorRain(byte red, byte green, byte blue, byte meteorSize, byte meteorTrailDecay, boolean meteorRandomDecay, int SpeedDelay) {  
  if (Strip1Position==0) { fill_solid( leds1, NUM_LEDS_PER_STRIP_A, CRGB(0,0,0)); }
  if (Strip2Position==0) { fill_solid( leds2, NUM_LEDS_PER_STRIP_B, CRGB(0,0,0)); }
 
  
  // fade brightness all LEDs one step
  for(int j=0; j<NUM_LEDS_PER_STRIP_A; j++) {
    if( (!meteorRandomDecay) || (random(10)>5) ) {
      leds1.fadeToBlackBy(meteorTrailDecay); 
    }
  }
  
  for(int j=0; j<NUM_LEDS_PER_STRIP_B; j++) {
    if( (!meteorRandomDecay) || (random(10)>5) ) {
      leds2.fadeToBlackBy(meteorTrailDecay); 
    }
  }
  
   
  // draw meteor
  for(int j = 0; j < meteorSize; j++) {
    if( ( i-j <NUM_LEDS_PER_STRIP_A) && (i-j>=0) ) {
      leds1 = CRGB(red, green, blue);
    }
  }
   
  for(int j = 0; j < meteorSize; j++) {
    if( ( i-j <NUM_LEDS_PER_STRIP_B) && (i-j>=0) ) {
      leds2 = CRGB(red, green, blue);
    }
  }

  
  FastLED.show();
  delay(SpeedDelay);
  
  if(NUM_LEDS_PER_STRIP_APosition<NUM_LEDS_PER_STRIP_A+NUM_LEDS_PER_STRIP_A) {
    Strip1Position++; 
  }
  else
  {
    Strip1Postion = 0;
  }
  
  if(NUM_LEDS_PER_STRIP_BPosition<NUM_LEDS_PER_STRIP_B+NUM_LEDS_PER_STRIP_B) {
    Strip2Position++; 
  }
  else
  {
    Strip2Position = 0;
  }
  
 
}
}

}