SQL Server 2014

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select @@version;

CREATE TABLE Stocks
(
  id int PRIMARY KEY IDENTITY(1,1),
  itemId int NOT NULL,
  qty int  NOT NULL,
   status NVarChar(50) NOT NULL,
)

CREATE TABLE Purchases
(
  id int PRIMARY KEY IDENTITY(1,1),
  itemId int NOT NULL,
  suppId int NOT NULL,
  qty int  NOT NULL,
  Date NVarChar(50) NOT NULL,
  status NVarChar(50) NOT NULL,
)

-- The Idea here is to Update in stock quantity if the itemid already existed 
--  and Insert if the itemId was not found when ever

-- Here is what I have tried so far

CREATE trigger [dbo].[trPurchaseInsert] on [dbo].[Purchases] FOR INSERT
 AS
 DECLARE @id int;
 DECLARE @itemId int;
 DECLARE @status NVarChar(50) = 'available';

SELECT @itemId=i.ItemId FROM INSERTED i
 IF EXISTS(SELECT * FROM Stocks WHERE itemId=@itemId)
 BEGIN
	SET NOCOUNT ON;

DECLARE @itemIdUpdate int;
	DECLARE @qty int;

SELECT @qty=[quantity], @itemIdUpdate=[itemId] FROM INSERTED i 
		
	UPDATE Stocks SET qty=qty+@qty WHERE itemId=@itemIdUpdate
 END
 ELSE
 BEGIN
	SET NOCOUNT ON;
        INSERT INTO Stocks SELECT [itemId], [id], [quantity], @status FROM INSERTED i
 END

-- The above Trigger works by:
--     Updating only the first itemId,  if itemId is found, the end there
--     inserts if no itemId is found

I have also tried Cursor even though its advised to avoid as much as possible

--Stored Proc
CREATE PROC [dbo].[spStockPurchase]
	 @itemId int, @purchaseId int, @qty int, @status NVarChar(50)
 AS
 IF EXISTS(SELECT * FROM Stocks s WHERE s.itemId=@itemId)
 BEGIN
		UPDATE Stocks SET qty=@qty, status=@status WHERE itemId=@itemId
 END 
 ELSE
 BEGIN
		INSERT INTO Stocks (itemId, purchaseId, qty, status)
		VALUES( @itemId, @purchaseId, @qty, @status)
 END

-- Trigger
CREATE TRIGGER [dbo].[trgTestPurchaseInsert] ON [dbo].[Purchases]
	AFTER INSERT
 As
 BEGIN

DECLARE @id int;
   DECLARE @itemId int;
   DECLARE @purchaseId int;
   DECLARE @qty int;
   DECLARE @status NVarChar(50) = 'available';
   DECLARE @counter INT
    
	SET @counter = 0;

DECLARE yucky_Cursor CURSOR FOR

SELECT itemId, id, quantity, status FROM INSERTED i;
	  OPEN yucky_Cursor;
	  FETCH NEXT FROM yucky_Cursor INTO @itemId, @purchaseId, @qty, @status;
	   WHILE @@FETCH_STATUS = 0 
   BEGIN
		DECLARE @rowCount int;
		 SELECT @rowCount = COUNT(*) FROM INSERTED;

WHILE @counter <= @rowCount
		BEGIN
				EXEC spStockPurchase @itemId, @purchaseId, @qty, @status;
				SET @counter = @counter + 1;
		END;
		FETCH NEXT FROM yucky_Cursor INTO @itemId, @purchaseId, @qty, @status;
   END
    CLOSE yucky_Cursor;
    DEALLOCATE yucky_Cursor;
  END

-- This one also updates the first Records if itemId is found and 
--  does nothing if itemId isn't foun

Msg 111 Level 15 State 1 Line 34
'CREATE TRIGGER' must be the first statement in a query batch.
Msg 102 Level 15 State 1 Line 66
Incorrect syntax near 'I'.
Msg 137 Level 15 State 2 Line 81
Must declare the scalar variable "@purchaseId".
Msg 134 Level 15 State 1 Line 90
The variable name '@id' has already been declared. Variable names must be unique within a query batch or stored procedure.
Msg 134 Level 15 State 1 Line 91
The variable name '@itemId' has already been declared. Variable names must be unique within a query batch or stored procedure.
Msg 134 Level 15 State 1 Line 93
The variable name '@qty' has already been declared. Variable names must be unique within a query batch or stored procedure.
Msg 134 Level 15 State 1 Line 94
The variable name '@status' has already been declared. Variable names must be unique within a query batch or stored procedure.
Msg 102 Level 15 State 1 Line 118
Incorrect syntax near 'END'.