I think it's not the problem of your job,but the table of the database. Maybe the table is readonly and you can't do any DML operation.
You have two options to resolve:
1. Remove the identity column from the schema (in the output of the tMap and propogate to the DBOutput component). Ensure that the DB output component has the "Turn on Identity Insert" set to off. SQL server takes care of the identity column and you just need to map all other columns in the schema.
2. Leave the identity column in the schema. Ensure "Turn on Identity Insert" is set to on in the DB output component and map an explicit value to the identity column (using a sequence).
Either option should work. Your choice.