There is no main category related to context: default
On the Github page there is an issue where a contributor suggests running this command:
php app/console sonata:classification:fix-context
The reason you get this error is because you're trying to add media under the context default but the context is not related (linked) to a category.
To fix sonata media contexts run this:
php app/console sonata:media:fix-media-contextas mentioned here