RuntimeError: You must setup() the GPIO channel first (4x4 Numpad on Raspberry Pi)

Question

I wanted to use my 4x4 numpad with my Raspberry Pi 3B+ and followed the instructions of this YouTube tutorial: https://www.youtube.com/watch?v=yYnX5QodqQ4 but I got the following error:

RuntimeError: You must setup() the GPIO channel first

This is my Python code:

import RPi.GPIO as GPIO
GPIO.setwarnings(False)
GPIO.setmode(GPIO.BOARD)

MATRIX =    [   [1,2,3,'A'],
                [4,5,6,'B'],
                [7,8,9,'C'],
                ['*',0,'#','D']     ]        

ROW = [7,11,13,15]
COL = [12,16,18,22]

for j in range(4):
    GPIO.setup(COL[j], GPIO.OUT)
    GPIO.output(COL[j], 1)

for i in range(4):
    GPIO.setup(ROW[1], GPIO.IN, pull_up_down = GPIO.PUD_UP)

try:
    while(True):
        for j in range(4):
            GPIO.output(COL[j],0)

            for i in range(4):
                if GPIO.input(ROW[i]) == 0:
                    print MATRIX[i][j]
                    while(GPIO.input(ROW[i]) == 0):
                        pass

            GPIO.output(COL[j],1)

except KeyboardInterrupt:
    GPIO.cleanup()

I tried to change GPIO.setmode(GPIO.BOARD) to GPIO.setmode(GPIO.BCM) and to remove GPIO.cleanup() but nothing worked. Does anyone have an idea how to solve this problem?

Thanks in advance!