If you've bound a datatable to an SQL table or some other ADO provider then I'm not sure you can merge in this way. As it tries binding the data in the table back to the ADO provider. And as the error message suggests, your merged data table fails the constraints imposed on the table.
you need to make sure the schemas for the data tables are very similar.
So you need to make sure the data table schemas are compatible. You then need to make sure that the data you're trying to merge is not going to break any of the constraints imposed by the database.