You have the clauses the wrong way; the default value has to come before the null/not null indicator:
CREATE TABLE PersonsNotNull
(
P_Id int NOT NULL,
LastName varchar(255) DEFAULT 'KHATTAR' NOT NULL ,
FirstName varchar(255),
Address varchar(255),
City varchar(255)
);
Table PERSONSNOTNULL created.
You're creating a relation table so the relation properties and column properties syntax apply:

From that you can see that the optional DEFAULT expr
clause comes before the constraints, which includes null/not null:
