#include <Nextion.h>
Regarding the syntax of communication to the Nextion there have been many reported issues with using "Nextion.h".
It would be better to use either the methods of Perry Bebbington documented in
https://forum.arduino.cc/t/using-nextion-displays-with-arduino/580289
or alternatively the EasyNextionLibrary by Seithan which is available through the library manager.
You will not find much support on this Arduino forum for using the ITEAD Nextion.h library.
I personally use the EasyNextionLibrary and find the trigger functions very helpful for sending values from the Nextion to the Arduino.