Sometimes students complete the assignment but forget the complete the last step i.e. click on the "Submit" button. Such students will show as "In Progress" in your class live board.
Unless students submit the work, teachers can not grade their work. You have several options to resolve this issue:
Ask the students to submit: If the student is available in class, ask him/her to sign-in, resume assignment and click on submit.
Wait until due date: Once the due date for an assignment has passed, student work in-progress would be auto-submitted. You can also close the assignment manually using the "Close" button. This will automatically force an auto-submit for all students.
Mark student as submitted: You can also select one or more students and mark them as submitted. This option is only available to teachers on the "Teacher Premium" or "Enterprise Plan".