WARNING: topic [/chatter] does not appear to be published yet

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I have a problem with a publisher node that does not publish in the desired topic. the idea is to retrieve image processing data and send them to the chatter topic

here is my python code:

import rospy
from std_msgs.msg import Float64
import numpy as np
import cv2
import serial

video_capture = cv2.VideoCapture(1)
video_capture.set(3, 320)
video_capture.set(4, 240)

while(True):
    flag, frame = video_capture.read()
    crop_img = frame
    gray = cv2.cvtColor(crop_img, cv2.COLOR_BGR2GRAY)
    blur = cv2.GaussianBlur(gray,(5,5),0)
    ret,thresh = cv2.threshold(blur,60,255,cv2.THRESH_BINARY_INV)
    hierarchy, contours, hierarchy = cv2.findContours(thresh.copy(), 1, cv2.CHAIN_APPROX_NONE)

    if cv2.waitKey(1) & 0xFF == ord('q'):
            break

    if len(contours) > 0:
            c = max(contours, key=cv2.contourArea)
            M = cv2.moments(c)
            cx = int(M['m10']/M['m00'])
            cy = int(M['m01']/M['m00'])
            cv2.line(crop_img,(cx,0),(cx,720),(255,0,0),1)
            cv2.line(crop_img,(0,cy),(1280,cy),(255,0,0),1)
            cv2.drawContours(crop_img, contours, -1, (0,255,0), 1)
            cv2.imshow('frame',frame)
            if cx >= 120:
                cmd=255
                print (cmd)

            if cx < 120 and cx > 50:
                cmd=1
                print (cmd)

            if cx <= 50:
                cmd=-255
                print (cmd)

            else:
                cmd=0
                print (cmd)
#ROS PUB
    def talker():
            rospy.init_node('opencvachraf_node', anonymous=True)
            rate = rospy.Rate(10) # 10hz
        while not rospy.is_shutdown(): 
            pub = rospy.Publisher('chatter', Float64,queue_size=3)

                 pub.publish(cmd)
             rate.sleep()


        if __name__ == '__main__':
                try:
                    opencvachraf_node()
            except rospy.ROSInterruptException:
                    pass