ESP8266 libraries impossible to upload in IOT CLOUD enviroment

Hello , I wish to use an ESP8266 board in OTA mode for enviroment sensors detection.
Is a project alredy made using thingspeak.
For this application I had purchased a plane for a best mode to show the data.
During the verify I get an error due to the ESP8266WIFI.H library:
"/tmp/532481373/ENVY_su_OTA_copy/ENVY_su_OTA_copy.ino:23:10: fatal error: ESP8266WiFi.h: No such file or directory"
I tried to install tha library but I can't find the right way.
This is the firs library then other are listed and I affraid to meet again the same problem.
Please, can someone give me an help?
Many thanks in advantage
Regards
Daniele

Hello

Did you install the ESP8266 Arduino core correctly ? Installing — ESP8266 Arduino Core 3.1.1-15-gbadb407b documentation