Assignments are optional. However, in order to receive a certificate of completion, you must submit and pass all assignments before the end of the course.
You can submit your assignment at any time as long as the course is running. Check the main course page to see when the course ends.
You can submit assignments in any order. This means you can submit them out of order if you wish.
What if the assignment is too difficult for me?
You can alter the assignment to make it easier for yourself. For example, try playing an easier left-hand part, or maybe just submit the melody alone. Point is, it is best to submit something rather than nothing.