This is the most straightforward error. It means the Arduino IDE's compiler cannot find the Blynk library.
: Try compiling the sketch by clicking the Verify button (checkmark icon). If the compilation completes without errors, your Blynk library installation is successful. blynksimpleesp8266 h library zip
Serial.println("Connected to WiFi"); Blynk.begin(auth, WiFi, ssid, password); This is the most straightforward error