Transact SQL :: ALTER Statement Is Not Getting Executed In A Batch

I have written the following code:
 
SET NOCOUNT ON
DECLARE @RowCount int; SET @RowCount = 0;
Begin Try
Begin Transaction
--------------------------------------------------------
-----Table Name: AlertsStaticRecord_Archive
-----Column Name: AlertID
--------------------------------------------------------
ALTER TABLE  [AlertsStaticRecord_Archive]  ALTER COLUMN [AlertID] int NOT NULL;

[Code] .....

But, when I execute these batch, I am getting error:

Msg 8111, Level 16, State 1, Line 11
Cannot define PRIMARY KEY constraint on nullable column in table 'AlertsStaticRecord_Archive'.
Msg 1750, Level 16, State 0, Line 11
Could not create constraint. See previous errors.

Because, the first ALTER statement is not getting executed.