Doesn't the error message seem pretty straight forward?
You declared an object of class RTC_DS3231 and then tried to call a member function .isrunning() which doesn't exist.
It does exist for the RTC_DS1307 class, but that is not what you are using.
It does have the .lostPower() function so you could use that to see if you need to set the time to the compile time or not.