First: Check inside the .mod file to ensure that the curly brackets are correct.
(In my experience, CK2 has a tendency to mangle the .mod file when it uploads to the workshop. This is not a problem for CK2 itself, but it makes the Validator very unhappy.)
Then: Make sure the dropdown near the top right of the Validator is set to "main mod errors".
(TBH I'm not 100% sure what the other options do, as I discovered a long time ago that this was the best choice for my purposes.)
Then: Try removing spaces, capitals and special characters from the .mod filename and all subfolder names.
(Eg: "greco-norman_and_the_sicilian_expedition.mod")
(This is probably a bit time-consuming, which is why I'm suggesting you try the other possibilities first!)
(Here, I'm not suggesting you change the human-readable name in the "name" property of the .mod file doesn't need to change - only the file- and folder-paths.)
(I think the most likely issue here is the "+" character, but testing every possibility one-by-one could take a lot of time, so why not throw everything at it?! Also, standardising on lowercase and "no spaces or weird characters" is a very useful coding convention IMO.)