Hi @mh0322
there is some incompatibility between these two libraries.
See, if you compile this empty sketch you will get the same error.
#include "WiFi.h"
#include "ESPAsyncWebServer.h"
void setup(){
}
void loop() {
}
Search the web to see if you can find any tips about this incompatibility.
PS: I FInd this:
For ESP8266 it requires ESPAsyncTCP To use this library you might need to have the latest git versions of ESP8266
RV mineirin