Write To Data Store Entity Error?

In order to write any row of data to a database table you need to assign a Primary Key to the row being inserted. If you look at the error you can see the row of data that is being attempted to be written, and the first value is <null>.

You can either make the assignment yourself, but why would you when you can ask the database to manage this for you. If you go to the database table definition you can mark the column that you want to use as the Primary Key to "Auto Increment" and the database will assign the value for you.