Hey Brandon, great question. UDIDs are not supported as device Identifiers for iOS devices so that is why you are noticing that error when trying to save them as test devices.
What we recommend using as a device identifier are Channel Ids, all devices have a persistent Id and they are assigned to every device that has the application installed. Here are some further details about Channels -https://docs.urbanairship.com/guides/channels/#addressing-channels and the Channels API -m https://docs.urbanairship.com/api/ua/#api-channels
Hopefully, that helps!