You can resubmit unless the author switched it off for that assignment. Resubmission is on by default. Where it is off, your first submission is your only one and a second is refused with "Resubmission is not allowed for this assignment."
Each submission is a numbered attempt, and the newest one counts
Resubmitting does not overwrite the old attempt, it adds a new one, and both grading and course completion read your most recent attempt.
That has a consequence worth acting on. A fresh attempt starts ungraded, and the assignment page shows the feedback on your latest attempt only. Copy any feedback you want to keep before you resubmit, because a new attempt pushes the earlier one, and its comments, out of view.
Submitting is also what marks the lesson complete. It ticks on submission, not on being graded and not on passing, and a low grade does not take the tick away.
What you can submit
The author chooses which formats an assignment accepts: a link, a file, a written essay, a UNI project, or some combination. Anything not on that list is rejected, and a project has to be one you are on the team for.
Who grades you
Three people can grade a submission, and any one of them is enough: the course author, your cohort's professor if you joined through a class code, and an Owner, Admin or Curator of the organization the course belongs to. A professor can only grade submissions from cohorts they run.
Grading runs against the assignment's rubric. Each criterion carries a score range and a weight, and your result is the weighted total as a percentage of the maximum. You see the written feedback but never who wrote it.
Peer grading changes where the mark comes from
An assignment runs in one of three modes. Instructor takes the grader's mark. Peer aggregate takes the median of your peer scores once the required number of reviews has arrived. Blended mixes that median with the instructor's mark at a weight the author set. Under the last two there is no grade until enough reviews land, so a peer-graded assignment can sit unmarked for a while. Peer review covers that side.
Late work is flagged, not refused
A due date only exists if you are in a cohort and the author put an offset on the assignment. It is your cohort's start date plus that many days. Submitting after it is accepted normally and marked late for you and for your grader. Nothing closes and nothing is rejected for lateness.
An assignment carrying a pass mark above zero has to be passed before the course counts as complete and your certificate issues. A pass mark of zero does not gate anything.