i removed the lines in bold :-) its adafruits guide thats wrong
void loop() {
delay(1000); // Wait 1 second between transmits, could also 'sleep' here!
char radiopacket[20] = "Hello World #";
itoa(packetnum++, radiopacket+13, 10);
Serial.print("Sending "); Serial.println(radiopacket);
if (radio.sendWithRetry(RECEIVER, radiopacket, strlen(radiopacket))) { //target node Id, message as string or byte array, message length
Serial.println("OK");
Blink(LED, 50, 3); //blink LED 3 times, 50ms between blinks
}
radio.receiveDone(); //put radio in RX mode
Serial.flush(); //make sure all serial data is clocked out before sleeping the MCU
}