It happened AGAIN, and I simply can't find anything wrong. This HAS to b e a Keynote bug.
Here's a Test file for you:
The file is based on the simple black template that comes with Keynote. And it only contains ONE slide, with ONE object. That object was copied over from another Keynote file I created a few months ago. (Note: I took Gary's advice and only copied the object, not the entire slide).
Even when I delete the object, save & reopen the presentation, it says "Lato Font is missing".
Did you create this Keynote file or receive it from another source?
I get this warning which indicates the file was created using this font, if the font is not present it will show this warning.
Once I downloaded and installed the Lato font set I did not get the warning and Keynote played the slide without issue.
Font downloaded from: Lato.com
I created the file, but during the creation of the file, the Lato font was not installed – I'm 100% sure.
The square shape was copied over from another presentation I created. And THAT presentation was using the Lato font (though NOT in the shape I copied over). For the new presentation, I don't use Lato and I also don't want to have to install it. I don't want to run around in circles, particularly not with installing fonts on iOS.
Anyway – the square only serves as a simple example, in order to illustrate the problem in a concise way. There's a lot more shapes that I want to copy from the old "Lato" presentation to this new one. NONE of those shapes have any fonts or text inside them, yet they still come up with the "font missing" problem.
I'm hoping that someone from the Apple Support team is reading this.
The square shape was copied over from another presentation I created. And THAT presentation was using the Lato font
Keynote often carries over unwanted attributes when using objects or slides from another presentation, fonts can be particularly problematic, they don't have to be present on a slide but a reference to a font is often retained. This is not ideal but its what happens.
It would do no harm to validate all your fonts using Font Book (select all the fonts then File > Validate fonts) removing a corrupt or duplicate font solves many issues.
If all else fails, you either use elements from the other presentation for convenience and install the font as I did and it all works, or you recreate the new presentation from scratch. The choice is yours.