Ah! In that case, one or more items is missing it's appropriate identifier for that distributor. Which is pretty normal for the first few weeks using the system. As more orders and invoices get ingested, it's able to piece all that together. But I bet some random FOC additions could be the culprit. But you need to add the missing identifier to the items missing one.
 
First off, if you reload the page, you'll have the ability to hit export again. Because it's a website, and so many of those actions touch a lot of others things while executing, a page refresh clears the field for it.
 
SO! The page it took you to was a big ole list of the items on your order. the left column is each items identifier for that distributor, the right column is the title and a link to the item. The items that are missing their identifier are the items on this list that don’t have anything in the column labeled “identifier” (the left column).
 
You’ll want to look up that item on your distributor's website and find that item’s identifier. Once you copy that Identifier from your distributor, (Identifiers on Lunar are the things that look kinda like a month-code. for example “1224IM830” is a Lunar identifier for an issue of Spawn. In this case the "IM" in it is referring to "Image" publishers) go back to your item in CSA, and under the “distros” tab you need to click the blue button that says “add distro”, select the appropriate distributor and paste the identifier into the field that says “Distributor SKU”. Once you’ve done that for each offending item, you should be able to export your file appropriately. (In some cases your distributor is already present on the list, and you just need to hit the "edit" icon next to it to add the identifier).
 
Hopefully there aren't too many items, but rest assured that the system is self-building, and after you get through this first round of missing identifiers, things get more streamlined.