If the integration to Salesforce is throwing errors with the following message "INVALID_FIELD_FOR_INSERT_UPDATE" it's likely because the object.property being passed through the API doesn't exist -- or if it does -- it isn't visible via permissions.
Frequently, we receive this in the NPSP accounts for the field, npsp__Primary_Contact__c.
The field exists in the "setup" -> "build" -> "customize" -> "opportunities" -> "Fields". But if you click on the field name, "Primary Contact" and then on "Set Field-Level security", you'll see that the permissions may be all un-checked.
Check appropriate roles (System Admin & Standard User) and if this field is visible through the API, you will no longer receive this error message.