The submission page on Bonfire requires a small handful of checks and balances. If a Requested Information (upload) slot is marked as Required, a document must occupy that slot. You must also check off the check box next to "I understand that I can't change any of the submission details or documents once the project closes".
Once all requested document slots that are required are fulfilled and that check box has been checked off, the "Submit & Finalize My Submission" button will become active.
The below screenshot shows the difference between a Required Slot that has been filled (top) and one that has not yet been filled (bottom).