I must admit that I am unable to fully comprehend your code, since you did not include the Main method, and it looks somewhat scanty.
Looking at the signature of your method i.e.,
public static long CalculateScore(byte chances, long millisec)
it is clear that all paths through the method must return a value of type long.
Please check the code to ensure all if-else, switch-case or other conditional statements always result in the method returning a value of type long.
Hope it helps.