I haven't used GraphQL with dotcms but I think your query needs to include the structure name with the field.
That's how it works in every other method to pull content from dotCMS, so I could only assume the same would apply here. The doc page for GraphQL does a bad job here because it only uses the title field for examples, which is sort of a special case in dotCMS that doesn't need the structure name included.
Hope that helps. As I said, I am not really sure because I've not used the GraphQL stuff at all.