the error is pretty clear. the DataSet class does not support column types of Nullable<T>. in your case System.DateTime?
In a DataSet when defining a column you use the actual type and set a property whether the column is nullable. When null, DbNull is returned for the column value.
You appear to be using a library that builds a DataSet based on POCO object types, but this library does not support Nullable<T> or it would unwrap the type and set nullable when creating the column in the DataSet.
basically your database adapter does not support nullable value types.