SQL Server Error: The multi-part identifier 'TBLLeaveEarned.Sick' could not be bound.

You need to select the Sick column value from the Database.

You cannot get the Sick value like this

TBLLeaveEarned.Sick.

Refer

DECLARE @leavecount INT
SELECT  @leavecount = SUM(TBLLeave.NoofDays)
FROM TBLLeave INNER JOIN TBLPersonalInformation ON TBLLeave.EmployeeID = TBLPersonalInformation.EmployeeID 
			  INNER JOIN RefLeaveStatus ON TBLLeave.Status = RefLeaveStatus.LeaveStatusID 
			  INNER JOIN TBLLeaveEarned ON TBLLeave.EmployeeID = TBLLeaveEarned.EmployeeID
WHERE (TBLLeave.EmployeeID = 'haha') AND (RefLeaveStatus.StatusName = 'Approved')
GROUP BY TBLPersonalInformation.Surname, TBLPersonalInformation.FirstName, TBLPersonalInformation.MiddleName, RefLeaveStatus.StatusName 
DECLARE @Sick INT
SET @Sick = (--Select the Sick value here. Use the SELECT Statement to get the Data) 
if(@leavecount < @Sick)
    BEGIN
        INSERT INTO TBLLeave(EmployeeID, [Date], [Year], LeaveID, NoofDays, Remarks, [Status]) VALUES('haha', '1/2', '2015', '1', '1', 'paleave', '1')
        SELECT 'INSERT'
    END
else
    begin
        SELECT 'You reach the limit'
    end