Add primary key as row number in SQL Server

I am trying to create a primary key for my table. Primary key would be the row number. First row should have seq_no 1, row n should have sequence number n and so on

Emp_demo2

emp_id    emp_name   emp_sal_K    emp_manager
1         Ali         200          2
20         Zaid        770          4 and so on

Query to add and update columns

Alter table emp_demo2
Add seq_no int
Update emp_demo2 set seq_no = row_number

Error

Invalid column name ‘seq_no’.

Answer

Use this line instead of add and update

Alter table emp_demo2 add seq_no int identity not null;