The Submit & Finalize My Submission will be unavailable for the following reasons:

• missing information for the required slots (denoted by the red dot)
• you have not agreed to the terms - "I understand that I can't change any of the submission details or documents once the project closes."

Once all required requested document slots are fulfilled and you have agreed to the terms, 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).

For more information on uploading a submission, please check out this article - Creating and Uploading a Submission.