Every booked unit needs artwork before it can run. The Upload section of the Creatives tab is where you supply it.
Uploading Artwork
Open the campaign and go to Creatives → Upload.
Find the units you are supplying artwork for.
Upload the creative file.
Getting The Specification Right
Out-of-home artwork is unit-specific. A file that is right for one board will be wrong for another, so check before you produce it:
Size. On the unit's detail view and the campaign grid.
Screen type. Static needs print-ready artwork; digital needs a digital asset, and may allow motion.
The media owner's spec sheet, where one is available for the unit.
*Note: Producing artwork before checking specifications is the most expensive mistake in this part of the process. Reprinting a large-format static is not cheap, and it eats into your installation window.
Working To Deadlines
Each unit has an artwork deadline, driven by how long the media owner needs to produce and install. Static formats need considerably more lead time than digital.
To see them all at once, add the Design Asset Due Date column to your campaign grid and sort by it. Much more reliable than opening units one at a time.
Finding What Is Missing
On your campaign's plan, the Creative Status filter shows units by whether artwork has been supplied. Filtering to those missing creative gives you your outstanding list directly.
Worth doing as a routine check in the run-up to launch, rather than relying on remembering which units you have done.