I am receiving an error when the integration runs from QuickBooks to Salesforce stating the following:

"INVALID_FIELD_FOR_INSERT_UPDATE: Cannot specify any additional fields when marrying or separating a Person-Account"

 What does this mean?

This error would be supplied in an instance where a Business Account is trying to be updated as a Person Account. Business Accounts and Person Accounts have different fields in them.

This can happen in instances where the Customer is synced to Salesforce as a normal account and, later, the Customer Type for that Customer is modified to the Customer Type specified in the DataSynch configuration for Person Account insertion.

If  a Customer in QuickBooks has the Customer Type listed as the same as what is listed for Person Account Type in the DataSynch configuration, and it is already linked to an Account in Salesforce (as a Business Account), the integration will attempt to insert/update the Account in Salesforce as a Person Account. However, since it is already linked to a Business Account it cannot be turned into a Person Account unless certain conditions are met. More information for that is available at here.

This error could also be applied where a Contact has been created for a QuickBooks Customer which is being synced to Salesforce.com as a Person Account. When a Person Account is created in Salesforce, a Contact may not be created for it since it is assumed that the Person Account is also the Contact. To get rid of this error, you would need to delete the contact information from the QuickBooks Customer where that Customer is being inserted/updated into Salesforce as a Person Account.