The course ‘Starts’ date and ‘Ends’ dates were set when your Canvas courses were created, and by default, “Students can only participate in the course between these dates” is also checked.
The ‘Ends’ date of a standard course is 21 January 2021.
And the ‘Ends’ date of a Final Assessment course is 20 July 2020.
The due dates in assignments and quizzes cannot be after term end (the Ends dates in course settings). So if you see an error message as shown in the screenshot below, go to Settings in your course, ensure the ‘Ends‘ dates are entered correctly, and the option “Students can only participate in the course between these dates” is checked. And remember to save the changes by clicking Update course details in the bottom of the page.