Not all code paths return a value

public static long CalculateScore(byte chances, long millisec)
{
int score;
byte num1 = chances++;
byte num2 = (byte)10;
byte num3 = (byte)10;
switch (Program.currLevelIndex)
{
case (byte)7:
num2 = (byte)10;
break;
case (byte)11:
num2 = (byte)20;
break;
case (byte)15:
num2 = (byte)40;
break;
}

if ((int)chances >= 1 && (int)chances <= 3)
num1 = (byte)40;
else if ((int)chances >= 4 && (int)chances <= 6)
num1 = (byte)20;
else if ((int)chances > 7)
num1 = (byte)10;
if (millisec > 480000L)
num3 = (byte)10;
else if (millisec >= 240000L && millisec <= 480000L)
num3 = (byte)20;
else if (millisec < 240000L)
num3 = (byte)40;
try
{
score = Convert.ToInt32((int)num2 * (int)num1 * (int)num3);
}
catch (Exception ex)
{
score = 0;
}

Console.Clear();

Console.SetCursorPosition(Program.x, Program.y);
Console.Write("Your Score was: " + score);
}


From this your codes above make sure you return like this
 public static long CalculateScore(byte chances, long millisec)
{
            int score;
byte num1 = chances++;
byte num2 = (byte)10;
byte num3 = (byte)10;

    return num1;
}