Publish to a Channel Using an Arduino Client | Arduino | Forum

The code that i am using is as per the Thingspeak web site. Apart from my debug Serial.print lines.
I have tried both of the mqttpublish() functions.
Both have no msgBuffer content.

Here is the Serial Monitor display
field1=22.1000000000&field2=43.9000020000&field3=512
topicBuffer=channels/3*****3/publish/A***********9
msgBuffer=

Why is the msgBuffer empty just prior to the upload ?
mqttClient.publish( topicBuffer, msgBuffer );code

Here the ThingSpeak code for 3 fields

void mqttpublish() {

float t = dht.readTemperature(); // Read temperature from DHT sensor.
float h = dht.readHumidity(); // Read humidity from DHT sensor.
//int lightLevel = analogRead(LIGHTPIN); // Read voltage from light sensor.
int lightLevel=512;//temporary test data

Serial.print(F("T="));Serial.print(t);Serial.println(F("C"));

// Create data string to send to ThingSpeak.
String data = String("field1=" + String(t, DEC) + "&field2=" + String(h, DEC) + "&field3=" + String(lightLevel, DEC));
int length = data.length();
char msgBuffer[length];
data.toCharArray(msgBuffer,length+1);
Serial.println(msgBuffer);

// Create a topic string and publish data to ThingSpeak channel feed.
String topicString ="channels/" + String( channelID ) + "/publish/"+String(writeAPIKey);
length=topicString.length();
char topicBuffer[length];
topicString.toCharArray(topicBuffer,length+1);

Serial.print(F("topicBuffer="));Serial.println(topicBuffer);
Serial.print(F("msgBuffer="));Serial.println(msgBuffer);

mqttClient.publish( topicBuffer, msgBuffer );

lastConnectionTime = millis();
}

here is the IDE compile
Sketch uses 19136 bytes (38%) of program storage space. Maximum is 49152 bytes.
Global variables use 591 bytes (9%) of dynamic memory, leaving 5553 bytes for local variables. Maximum is 6144 bytes.

from the Arduino site
https://www.arduino.cc/reference/en/language/variables/utilities/progmem/

The F() macro

When an instruction like :

Serial.print("Write something on the Serial Monitor");

is used, the string to be printed is normally saved in RAM. If your sketch prints a lot of stuff on the Serial Monitor, you can easily fill the RAM. If you have free FLASH memory space, you can easily indicate that the string must be saved in FLASH using the syntax:

Serial.print(F("Write something on the Serial Monitor that is stored in FLASH"));