Pay attention to the table at the top which explains the different phases of the activity.
See if there's a green check mark next to the phrase "Submit your work" in the "Submission phase" section. If yes, you have successfully submitted your work. You can click the "Submit your work" link to see your submission, and you can edit it before the deadline if you'd like to. If you don't see a green check mark next to "Submit your work", you have not submitted your work.
If you have uploaded a document as part of your submission, make sure that it is in a proper file format (eg, an MS Word file). It is always a good idea to download what you have submitted and then to open it to check (a) whether the file opens and (b) if it has the right contents.