RDM6300 library not working on ESP8266
What fixed the issue for me was to go to Boards manager on Arduino IDE and update to latest version:

Then I got the following error when compiling:

Board generic (platform esp8266, package esp8266) is unknown

In order to fix that, I followed these instructions.
And, I also removed rdm6300 library from C:\Program Files\Arduino\libraries and moved it in C:\Users\myusername\Documents\Arduino\libraries