If you like to use Interstitials you will have to create a matching Ad Type. To match the DFP "creativeSizeType" Interstitial do the following:
- Pick the value "Out-of-page" for your slot size.
If you do not have this picklist value in your Org please have your administrator create it and assign it to the record type DFP - but make sure to follow the correct spelling "Out-of-page".