I can't get parent page query in wagtail
get_parent is a method on a specific instance of the Page class, it's not a property. You should not call it from the Page class directly, but rather from a specific page.
It should look something like this:
parent = MangaPage.objects.get(title='Some Title').specific().get_parent()
Essentially what you are doing in your code example is searching for a MangaPage that has a title equal to the method Page.get_parent, which of course there are none. You need to get the parent of a specific page instance.
I have tried this code but always return an empty query, and i don't know why.
parent = MangaPage.objects.filter(title=Page.get_parent)