Stored Procedure for blocking Draft Returns if a UDF is blank

I created a UDF RetReas (Return Reason) for Sales AR > Return. The UDF is applied to the rows. I need to block the adding of drafts if the UDF is blank. My code on the stored procedure, SBO_SP_TransactionNotification is not working. The draft for Return is still being added even if the UDF is blank. Here is the code

IF (@object_type ='112' AND @transaction_type IN ('A','U'))  
    BEGIN
	DECLARE @RetReason nvarchar(30)
        SELECT  @LineNum = b.VisOrder+1, @ObjType = CAST(A.ObjType as varchar(32)), @RetReason = COALESCE(B.U_RetReas,'')
        FROM ODRF A  INNER JOIN DRF1 B ON A.DocEntry = B.DocEntry
        WHERE A.DocEntry = @list_of_cols_val_tab_del and ISNULL(A.WddStatus,'-') <> '-'  
		IF (@ObjType = '16')
			IF ( @RetReason = '')
				BEGIN
					SET  @error =  -100018
					SET  @error_message=N'Return reason is required!'
				END

        
    END