In order work with Native Ads your Administrator and/or Inventory Manager has to set up Ad Types Slot Sizes accordingly: 

Administrator Tasks

Add Picklist Values

Open the Ad Type object and edit the values for the dropdown fields for 

1. ADvendio__Master__c and
2. ADvendio__CompanionSize__c

Create one or more new picklist values with the following syntax: "native (Google Ad Manager Creative_Template_Id)":  Native in lower case letters + blank + parenthesis + Google Ad Manager Creative Template ID + parenthesis. Example: native (10099414). Make sure that you only select ADMANAGER as a record type if you are using more than one adserver.

You can view these IDs in the Google Ad Manager UI under Delivery > Native > Native Formats (see the ID below each native ad format name in the table). 

Inventory Setup

Modify Ad Type Records

Next add those new values to the respective Ad Type records slot sizes which allow native ads. 

Translate Creative Template IDs (Optionally)

If needed you can also translate the IDs in the translation workbench to make them more readable for the end users. Example:

Read more 7.1.7 Localisation and Translation