Add a column with Not Null
As you already have data in the table, you cannot add a NOT NULL field, because the existing data will already violate the NOT NULL constraint.

You therefore have 2 choices:

a) Add the column with a DEFAULT value...

ALTER TABLE dbo.Tmp_tbl_post_category ADD NgoId INT DEFAULT 1;


...then run an update statement to correct the values to what they should actually be

b) Truncate the data from the table, add the column and then re-populate the table...

TRUNCATE TABLE dbo.Tmp_tbl_post_category;

ALTER TABLE dbo.Tmp_tbl_post_category ADD NgoId INT;

INSERT INTO dbo.Tmp_tbl_post_category...