In the course settings, is this particular course set to be in the "K12 Semester 1 - 2019-2020" term? I would confirm that first. The error message indicates that the problem with the assignment date settings is specifically the term dates, so the date in question is coming from the term setting for the course.
It is possible to set an end date for the course within the course settings, and this overrides the term end date (see How do term dates, course dates, and section dates work in Canvas? ), but since it looks like the term end date for"K12 Semester 1 - 2019-2020" should be fine for the assignment dates you're trying to use, that suggests that the course may be assigned to the wrong term.