If you are trying to submit an assignment but the Submit button remains inactive, first check the Dates tab in your course.
If it is a self-paced course and you have missed the suggested due date on an assignment, you will not be able to submit the assignment until you shift your due dates. You can read more about self-paced courses in this article.
If it is an instructor-paced course and you have missed the due date on an assignment, you will no longer be able to submit and the deadline remains fixed. You can read more about instructor-paced courses in this article.
If the course has ended and is now archived, the submit button may no longer work. You can read more about archived courses here.