I have this in the Preferences/boards manager:
http://arduino.esp8266.com/stable/package_esp8266com_index.json
This the error I get when trying to upload a sketch:
Arduino: 1.8.9 (Windows 7), Board: "LOLIN(WEMOS) D1 R2 & mini, 80 MHz, Flash, Disabled, All SSL ciphers (most compatible), 4M (no SPIFFS), v2 Lower Memory, Disabled, None, Only Sketch, 921600"
Board d1_mini (platform esp8266, package esp8266) is unknown
Error compiling for board LOLIN(WEMOS) D1 R2 & mini.
This report would have more information with
"Show verbose output during compilation"
option enabled in File -> Preferences.