Cannot insert the value NULL into column 'ID'

Hi dear experts:

The message is

"Cannot insert the value NULL into column 'ID_Month', table 'Database_DW.dbo.Incident_Summary'; column does not allow nulls..."

At my table, I have a primary key which make autonumber for any record set in column 'ID_Month'. How can I  insert autonumber in column 'ID_Month'by SQL statement?

Thank you in advance.