error in code i don't understand

so i was programming a speaker but ones trying to upload the code i got this error
Arduino: 1.8.7 (Windows 10), Board: "WeMos D1 R2 & mini, 80 MHz, 921600, 4M (3M SPIFFS)"

sketch_poepnoep_speaker:6:21: error: ESP8266.h: No such file or directory

Did you find this code somewhere on the Internet? If so, please post a link to it.

As the error indicates, there is a missing file named ESP8266.h. I'm not familiar with a file of that name so hopefully the person who wrote the code has provided some documentation to indicate where it might be procured.

Did you add ESP8266 library to you libraries. after adding to Arduino you should include it to your sketch.
See this link :
Library Managment

Maybe you did not add the library to your project.
You need to install it properly and the select also the proper target board for your project.

Silvano