[Django] error

OS	Ubuntu 16.04.4 LTS
VERSION	python 3.5.2
	Django 2.1.2

   요약

• TypeError: __init__() missing 1 required positional argument: 'on_delete' -> ERRORS:bookmarks.Bookmarks.user: (fields.E300) Field defines a relation with model 'User', which is either not installed, or is abstract.bookmarks.Bookmarks.user: (fields.E307) The field bookmarks.Bookmarks.user was declared with a lazy reference to 'bookmarks.user', but app 'bookmarks' doesn't provide model 'user'.