You are permitted to take a midterm (or other non-final) exam only after all assignments preceding the exam have been submitted and after all but the assignment prior to the midterm have been graded. For example, if there are three assignments preceding a midterm, all three must be submitted, and the first two must be graded.
Likewise, you are permitted to take a final exam only after all midterms (or other non-final exams) have been graded, after all assignments preceding the exam have been submitted, and after all but the assignment prior to the final have been graded.
Some courses may have stricter limitations on when exams may be taken. These restrictions will be listed in the course Introduction.