I know that I am having a problem where I use the RAISERROR Function. This was working until I put in RAISERROR(). We are required to use RAISERROR() and TRY / CATCH in the calling SQL Script of the below stored procedure. I am not sure what I should do at this point.

This part might be a problem:

IF NOT EXISTS (SELECT InvoiceNumber, InvoiceDate, InvoiceTotal,
		(InvoiceTotal - CreditTotal - PaymentTotal) AS Balance
	FROM Invoices
	WHERE InvoiceDate BETWEEN @DateMin AND @DateMax AND
		(InvoiceTotal - CreditTotal - PaymentTotal) > 0
	ORDER BY InvoiceDate)
	
	RAISERROR ('Invalid something here', 11, 1))

Here's the whole enchilada (Stored Procedure) and also its calling script:

-- 1. Create a stored procedure named spDateRange that accepts two
-- parameters, @DateMin and @DateMax, with data type VARCHAR and
-- default value null.
--
-- If called with no parameters or with null values, the procedure
-- should return an error message describing the syntax.
-- RAISERROR and TRY…CATCH must be used.

-- Write a script to call the procedure spDateRange.
-- If called with non-null values, validate the parameters.
-- Test that the literal strings are valid date ranges and
-- test that @DateMin is earlier than @DateMax.
--
-- If the parameters are valid, return a result set that includes
-- the InvoiceNumber, InvoiceDate, InvoiceTotal, and Balance
-- for each invoice for which the InvoiceDate is within the date
-- range, sorted with earliest invoice first.
--
-- The result set should contain invoices with balance only.
--
-- Use the following test data in the calling program to produce
-- 4 results:
USE AP
GO

ALTER PROCEDURE spDateRange
	@DateMin VARCHAR(20) = NULL,
	@DateMax VARCHAR(20) = NULL
AS

-- Declare Error Variables.
DECLARE @NullDateError INT,
	@BadDateError INT,
	@MinDateLargerThanMaxDateError INT

-- Initialize Error Variables.
SELECT 
	@NullDateError = 1,
	@BadDateError  = 2,
	@MinDateLargerThanMaxDateError = 3
	
-- Test for one or both values containing NULL. IF either value
-- contains NULL, there is no way to continue this stored procedure.
-- If any of the following errors occurs, the stored procedure stops.
IF @DateMin IS NULL OR @DateMax IS NULL
	RETURN @NullDateError

IF ISDATE(@DateMin) = 0 OR ISDATE(@DateMax) = 0
	RETURN @BadDateError

IF CAST(@DateMin AS DATETIME) > CAST(@DateMax AS DATETIME)
	RETURN @MinDateLargerThanMaxDateError

-- If you've gotten this far, then the MIN/MAX dates are good.
IF NOT EXISTS (SELECT InvoiceNumber, InvoiceDate, InvoiceTotal,
		(InvoiceTotal - CreditTotal - PaymentTotal) AS Balance
	FROM Invoices
	WHERE InvoiceDate BETWEEN @DateMin AND @DateMax AND
		(InvoiceTotal - CreditTotal - PaymentTotal) > 0
	ORDER BY InvoiceDate)
	
	RAISERROR ('Invalid something here', 11, 1))

-----------------------------------------------------------
-- T-SQL SCRIPT to Call the Stored Procedure, spDateRange.
-----------------------------------------------------------
DECLARE @Return INT
BEGIN TRY
	EXEC @Return = dbo.spDateRange '07/40/08', '08/01/08' -- Invalid Date.
	IF @Return = 0
		BEGIN
			PRINT 'Successful! Invoice Balances were found!'
		END
END TRY

BEGIN CATCH
	PRINT 'An error occurred.'
	PRINT 'Message: ' + ERROR_MESSAGE()

	IF @Return = 1
		PRINT 'INVALID DATE! One or both dates had a NULL value.'

	IF @Return = 2
		PRINT 'INVALID DATE: One, or both, of the date parameters is invalid.'

	IF @Return = 3
		PRINT 'Parameter [@DateMin] was greater than [@DateMax].'
END CATCH
----------------------------------------------------------------------------
-- Other calls that can be used to demonstrate all of the above scenarios:
EXEC spDateRange NULL, '08/01/08'		-- Test for one value containing NULL.
EXEC spDateRange '08/10/08', '07/10/08' -- Test for Min Date larger than Max Date.
EXEC spDateRange '07/40/08', '08/01/08' -- Test for an invalid date.