if NOT EXIST does not work for the select statment in the stored procedure even if the field is blank

if "NOT EXIST" is the type of code that for some reason  does not work for the select statment in the stored procedure even if the field is blank

Can you help write the correct code....

SELECT @user=UserId FROM tableName id = @id
IF NOT EXISTS(SELECT [test] FROM tableName WHERE  id = @id)
BEGIN

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