The "Hand in"-button is only activated when you have uploaded a paper and all the required frontpage information fields have been filled out.
Please note that some frontpage information such as problem statement, must be filled in before you are able to hand in.
Likewise, one must confirm the "Solemn Declaration" in order to hand in if this field is required frontpage information.
Submission of a paper is only complete when you have pressed "Hand in" and the Hand in state is set to "Handed in", seen with green. Example below.