What is the syntax to create a table variable?

It should be either

CREATE TABLE username (name nvarchar(50))            

or

DECLARE @username TABLE (name nvarchar(50))            

As it is not possible to modify a Table Variable once created, and it can only have one index - the Primary Key - we always [well, nearly always!] add a PK in the declaration:

DECLARE @username TABLE            
(            
    [name] nvarchar(50) NOT NULL,            
    ... any other columns ...            
    PRIMARY KEY            
    (            
        [name]            
    )            
)