Okay, so there’s a couple things here. First is, the “media” import type is useful only for adding media to already-existing items. If that’s what you want to do, that’s good. If you want to instead create new items and media at the same time, you probably want to do an “Item” import. You can still pick “Media source” when importing items, and it will create the media and attach it to the newly-created item for the same row. Doing the import this way from the start is definitely a simpler way to get items and media created from a single CSV.
Now, about doing a “Media” import like you’ve shown here. That type of import just creates (or updates) Media objects, so it can’t create new items, it can only add to existing ones. It can make sense to use when you’ve got items and their metadata created already, and you just need to add the media.
First, to create new media you want to have the “Action” in Advanced Settings set to Create. The “Append” action for a media import would be used instead to modify existing media (to change Media metadata, for example).
Second, when you’re doing a Media import, there are two mappings you need to have, at a minimum. Those are “Item” (data that identifies the parent item the new media should be created under) and “Media source” (the URL, sideload filename, etc.). You have them both here in your screenshot, but where I think you’re going wrong is that you have mapped both of those to the same column. That’s pretty unlikely to be the right thing to do.
When you map “Item” and pick “Internal id” you’re saying that the data in the CSV column you’re mapping to contains the Omeka S ID number of the item you want to use as the parent. Instead it looks like “Photo 1” is probably a filename. It’s an appropriate column for mapping to “Media source” but probably not for “Item.”
What you need to pick here and which column you should use is dependent on your data, but “dcterms:identifier” is a common choice (so, if when you imported the items the first time you had a unique identifier in the CSV that you mapped to dcterms:identifier, then you could use that same column here and map it as “Item [dcterms:identifier]” to match up your imported media with the corresponding items you already imported before from the same rows.
