Alter table and update in sql server transactions
You have to break it up into batches because the [Column] does not exist since the statements are being parsed as one batch so it cannot see the new column, hence you receive the error.

Once you break it up into batches with GO statement the column will be added to the table and then your next batch updating the new column will be successful.

Full example:

DROP TABLE #A;
GO
CREATE TABLE #A (ID int IDENTITY(1,1), column1 varchar(1))
GO
INSERT INTO #A (column1)
VALUES('a'),('b'),('c')
GO

begin transaction
alter table #A add [Column] int;
go

begin transaction
update #A set [Column] = 1 where ID=1;
select * from #A;

rollback transaction;
GO

SELECT *
FROM #A