Ensure your power source provides at least 500mA. The ESP8266 draws significant current during the WiFi handshake.
Use the Boards Manager, not the Library Manager. Header: Always use #include . esp8266 wifi.h library download
Use this minimal code to test if your library is installed correctly and connect your board to the internet. Ensure your power source provides at least 500mA
Ensure your router is broadcasting a 2.4GHz signal; the ESP8266 does not support 5GHz. the ESP8266 does not support 5GHz.