If it's not a nullable column, you have to insert some value to existing table in SQL server.
To add a column in a table, use the following syntax:
ALTER TABLE table_name
ADD column_name datatype NULL DEFAULT default_value WITH VALUES
Keep in mind that if the column is nullable, then null will be the value used for existing rows.If it's not a nullable column, you have to insert some value of that data type. So, for existing records, new value will be inserted in them.
ALTER TABLE tbl_employee
ADD FAX varchar(50) NULL DEFAULT 'NOT AVAILABLE' WITH VALUES