This error indicates that you did an update of ESP8266 core for Arduino via Boards Manager but the Arduino IDE failed to completely remove the previous version of the package.
- Click the link at the line following File > Preferences > More preferences can be edited directly in the file. This will open the Arduino15 (or similar name) folder.
- Navigate to Arduino15/packages/esp8266/hardware/esp8266
- Here you should see two different version folders, one of which is the previous version you updated from. Delete that folder. Please be very careful when deleting things on your computer. When in doubt, back up!
After doing that the problem should be solved.