Ramsing:
Board generic (platform esp8266, package esp8266) is unknown
This error is caused by having multiple versions of the esp8266 package installed. This can occur if you update to a new version of the package but the Arduino IDE fails to completely remove the last version. Please do this:
Tools > Board > Generic ESP8266 Module
File > Examples > ESP8266 > Blink
Sketch > Show Sketch Folder
Move up folder levels until you get to the esp8266 folder. Do you see multiple folders? If so, what are they?