Your media file (.png) is offline.
I mean, its not linked to your composition.
Please right click on that file, in the project panel, select Replace footage>File and locate that .png file again.
Just as you replied, I spotted the same!
BUT - I have 279 "footage files" missing...I am just working out how to re-link them. :/
If they're all in the same place AE will find them all when you show it where one is. This is one of the reasons why it's a good practice to keep all the graphics files for a said project neatly together. It makes it easier to hand off to someone and if you run into the issue you're having here it's a simple fix. If Those 279 files you're missing are scattered all over the place I think you may be in for some tedious work. Or you could check out the menu options under File>Dependencies
Thank you. I am working on being "tidy" with the source files.
Everytime I start a project I create a project folder with at least two subfolders: Graphics & Audio.
I pin the graphics folder to Explorer's (windows) quick select menu and also to the "Favorites" menu in Bridge.
When I download photos off the net i immediately put them in the graphics folder via Explorers quick select menu.
When browsing my own image libraries on my computer I always do it in Bridge. When I find something I want to use I just right click it and choose Copy To> Graphics
Then of course everything I bring onto the project I drag from or import from that graphics folder.
This way everything for the project is all in one place.
I also learned the hard way to work like this. It takes an extra few seconds to move images before using them but it's worth it.