esp8266 v7 iii

You are trying to program an Uno with an ESP8266 connected to it for use as a WiFi module. For that application the ESP8266 core is of no use. For your application it's most common to run the AT firmware on the ESP8266 and communicate via serial.

Digital IO ¶. Pin numbers in Arduino correspond directly to the ESP8266 GPIO pin numbers. pinMode, digitalRead, and digitalWrite functions work as usual, so to read GPIO2, call digitalRead(2). Digital pins 0—15 can be INPUT, OUTPUT, or INPUT_PULLUP. Pin 16 can be INPUT, OUTPUT or INPUT_PULLDOWN_16.

For learning, testing, and prototyping, you'll want to use ESP8266 NodeMCU development boards. These come with all the needed circuitry to apply power, upload code, easy access to the GPIOs to connect sensors and actuators, an antenna for the Wi-Fi signal, and other useful features.

If you are eager to connect your new ESP8266 module to a Wi-Fi network to start sending and receiving data, this is a good place to start. If you are looking for more in depth details of how to program specific Wi-Fi networking functionality, you are also in the right place.