I am trying to build a custom bot with a few different sensors publishing to nodes and using Rviz to visualize said nodes. I get this error in Rviz. Transform[sender=unknown_publisher] For frame[]:Fixed Frame [map] does not exist Now i am super new to Ros and Rviz so any guidance would be greatly appreciated.

here is the code

#!/usr/bin/env python3
# coding: latin-1

import RPi.GPIO as GPIO
import rospy
import time
import math
import numpy
import tf
from std_msgs.msg import Float64
from sensor_msgs.msg import Range, LaserScan


trigPin = 16
echoPin = 18
MAX_DISTANCE = 220          # define the maximum measuring distance, unit: cm
timeOut = MAX_DISTANCE*60   # calculate timeout according to the maximum measuring distance

range_msg = Range()
range_msg.radiation_type = 0
range_msg.field_of_view = 15
range_msg.min_range = 2
range_msg.max_range  = 400




def pulseIn(pin,level,timeOut): # obtain pulse time of a pin under timeOut
    t0 = time.time()
    while(GPIO.input(pin) != level):
        if((time.time() - t0) > timeOut*0.000001):
            return 0;
    t0 = time.time()
    while(GPIO.input(pin) == level):
        if((time.time() - t0) > timeOut*0.000001):
            return 0;
    pulseTime = (time.time() - t0)*1000000
    return pulseTime

def getSonar():     # get the measurement results of ultrasonic module,with unit: cm
    GPIO.output(trigPin,GPIO.HIGH)      # make trigPin output 10us HIGH level
    time.sleep(0.00001)     # 10us
    GPIO.output(trigPin,GPIO.LOW) # make trigPin output LOW level
    pingTime = pulseIn(echoPin,GPIO.HIGH,timeOut)   # read plus time of echoPin
    distance = pingTime * 340.0 / 2.0 / 10000.0     # calculate distance with sound speed 340m/s
    return distance

def setup():
    GPIO.setmode(GPIO.BOARD)      # use PHYSICAL GPIO Numbering
    GPIO.setup(trigPin, GPIO.OUT)   # set trigPin to OUTPUT mode
    GPIO.setup(echoPin, GPIO.IN)    # set echoPin to INPUT mode

def loop():
    while(True):
        distance = getSonar() # get distance
        time.sleep(1)

if __name__ == '__main__':     # Program entrance
    print ('Program is starting...')
    rospy.init_node('ultrasonic_trans')
    pub = rospy.Publisher("/ultra", Range, queue_size=10)
    rate = rospy.Rate(1)
    setup()

    while not rospy.is_shutdown():
        range_msg.range  = getSonar()
        pub.publish(range_msg)
        rate.sleep()

    rospy.loginfo("Ultra Sonic Pub Closed")