In order to add a document to the missing task, you need to access the checklist for this transaction, find Action button at the top of the screen and select Add task from the drop down menu.
After that enter appropriate information and make sure you checked required box for the document to be attached.
All tasks within a checklist must be assigned to a user who has access to the transaction to be able to complete it. If the person you are assigning a task to is not listed under Users & contacts, the system will automatically add it for you. In order to assign any task, the user must have access to the transaction and be listed under Users & contacts. When the agent is removed from the transaction, all of the agent's tasks will be automatically re-assigned to the transaction owner.
If you don't know who a task should be assigned to, just keep this task assigned to the transaction owner. In case you change the transaction owner, all tasks assigned to the previous owner will get automatically re-assigned to the new Transaction owner.
This brief video gives 2 options on how to add documents to a checklist when appropriate tasks are missing.